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BiQelow, Maurice Alpheus,
IntroductJan to biology ; an elementarv
3 6105 04929 763d
SCHOOL OF EDUCATION
LIBRARY
TEXTBOOK
COLLECTION
m
J
STANFORD \^^ UNIVERSITY
LIBRARIES
DEPAETMEKT Of ia)UCATION
EELAHD STANFORD JUNIOR UNIYERSITY
INTRODUCTION TO BIOLOGY
THE MACMILLAN COMPANY
NBW YORK • BOSTON • CHICAGO • DALLAS
ATLANTA • SAM FRANCISCO
MACMILLAN & CO., Limited
LONDON • BOMBAY • CALCUTTA
MBLBOURNB
THE MACMILLAN CO. OF CANADA, Ltd.
TORONTO
INTRODUCTION TO BIOLOGY
AN ELEMENTARY TEXTBOOK AND
LABORATORY GUIDE
BY
MAURICE A. BIGELOW, Ph.D.
PROFESSOR OF BIOLOGY IN TEACHERS COLLEGE
COLUMBIA UNIVERSITY
AND
ANNA N. BIGELOW, M.S.
TBAOHBR OF BIOLOGY IN MISS CHAPIN*S SCHOOL
FOR GIRLS, NEW YORK
THE MACMILLAN COMPANY
1913
AU rights reeerved
626171
OoplifaaT, 1918,
By the MACMILLAN COMPANY.
Set up and electrotyped. Published September, 1913.
J. S. Oushiog Oo. — Berwick & Smith Gq.
Norwood, MasB.f U.S.A.
PREFACE
This elementary textbook and laboratory guide is intended
for use in the first or second year of the high school course.
The authors' " Applied Biology " is better adapted to students
more advanced than the second year of high school, and even
for the second year in some schools in which there is need of
more supplementary reading than this " Introduction " provides.
This book is not a combined textbook of botany .and zoology,
for it makes no attempt at a systematic presentation of either
of these sciences. It is simply what its title suggests; an
introduction to hiologicsl facts and ideas. These have been
selected with reference to their direct bearing on the daily
-life of intelligent citizens. In short, this is an introduction
to biology considered as science applicable to human life espe-
cially in economic or practical and hygienic lines, with limited
attention to facts and ideas whose applications are aesthetic and
-intellectual (e.g, evolution).
The authors have aimed to make this book an intrpdusefcion
to the usual courses in botany and zoology. Necessarily, some
of the materials for study are the same as those used in such
courses; but, on the whole, this book ought to prepare for,
rather than wastefully to anticipate, strong courses of "botany
or zoology which in many high schools will probably be offered
as electives in later years.
While recognizing that at present a large number of studielnts
in high schools are quite ignorant of the most elementary
nature-study such as many schools teach well even- m tlie
VI PREFACE
primary grades, it has seemed best to avoid giving prominence
to nature-study in this high-school book. There has long been
a tendency towards making a large part of courses of biology in
the first year of some high schools little more than nature-study
suitable for fourth to seventh grades. The authors have failed
to find high schools in which such nature-study is made difficult
or dignified enough to hold the attention and the respect of the
students as do the other sciences; and this has led to the
opinion that a very limited amount of nature-study for the sake
of general acquaintance with the common organisms and their
most striking habits should supplement, rather than form an
integral part, of high-school courses of biology. The authors
believe that, for high-school students^ a general knowledge of
the structure, and especially of the functions, of a few well-
selected animals and plants viewed from the standpoint of
applied science is to the average citizen more useful than
nature-study information (especially names, habits, and other
interesting facts) concerning dozens of common organisms.
For instance, an understanding of the respiratory machinery
and its workings in a seed-plant, an insect, a fish, an earth-
worm, and d lung-breathing animal is worth more to the aver-
age intelligent person than knowing the names of numerous
common insects, trees, and' birds.
The appendix contains a table for ready reference to sec-
tions of .the authors' " Applied Biology " and its accompanying
" Teachers' Manual of Biology." In these books will be found
many supplemental facts and references that will be useful to
teachers of classes that use this " Introduction."
As^ in the " Applied Biology," the authors have thought it
best to make extensive use of the standard illustrations which,
although more or less familiar to well-trained teachers, are
quite new to the students. Moreover, such illustrations seem
to have a decided advantage in having been prepared originally
by or under the direction of specialists. It is obviously im-
PREFACE VU
possible for the author of a general textbook, touching here
and there over the wide field of biology, to improve upon the
many classical figures whose obvious merits have caused them
to be chosen for various textbooks. Having this view of the
value of certain standard figures, the authors have chosen to
use them, even in some cases when new illustrations were
available and offered the possible attractiveness of novelty to
some teachers.
M. A. B.
A. N. B.
Teachers Collbge, Columbia Uniyebsitt
January 28, 1913.
CONTENTS
CHAPTBB PAOB
I. Preliminary Studies of Living and Lifeless
Things 1
IL The Characteristics of Living Things ... 10
III. First Lessons in Plant Biology .... 23
IV. The Work of the Organs of a Plant : Intro-
duction TO Plant Physiology .... 41
V. Study of Insects : Introduction to Animal Biol-
ogy 78
VI. Some Animal Studies Preliminary to Human Bi-
ology : Vertebrates 98
VII. Some Animal Studies Preliminary to Human Bi-
ology: Simple Animals 131
Vin. Human Structure- and Life-activities . . . 147
IX. Biology applied to Personal Hygiene . . . 223
X. Organisms that affect Human Health . • . . 256
XL Economic Relations of Organisms .... 295
XII. Reproduction of Organisms 368
APPENDIX: Table for Reference to the "Applied Bi-
ology " AND the " Teachers' Manual of Biology" 415
INDEX 418
INTRODUCTION^ TO BIOLOGY
CHAPTER I
PRELIMINARY STUDIES OF LIVING AND LIFELESS
THINGS *
1. Biology, Botany, and Zoblogy. — Biology is the science
that treats of life and all living things — plants, animals,
and man. There are two subdivisions of the science;
namely, zoology, treating of animals, and hotany, treating of
plants. Zoology is often called animal biology, and botany,
plant biology.
The division of biology into botany and zoology does not
mean that plants and animals are entirely imlike; but,
on the contrary, we shall find later that these two kinds of
living things have many points of remarkable similarity
in both structure and activities. This similarity is espe-
cially striking in many microscopic living things which so
combine both plant and animal characteristics that biologists
have not decided whether they are animals or plants. But
although there is often a great similarity between animals
and plants, it is sometimes convenient to study the two kinds
of living things separately, and so biological science is sub-
* To Teachers : This chapter contains subject-matter which is needed
in the study of biology, but with which few students in the early years of
the high-school course will be familiar. Many teachers will prefer to omit
§§ 5-29, and refer to them when they are directly applicable to the inmie-
diate lesson, as suggested in a foot-note to § 60.
B 1
2 INTRODUCTION TO BIOLOGY
divided into botany and zoology. These two subdivisions
of biology are most important for advanced students; but
the best and most interesting beginning study is that which
directs attention to the great facts common to all living
things. This book for beginners is called '' Biology," to in-
dicate that it uses both animals and plants to illustrate facts
and ideas which are true of all living things.
More important than the similarity of animals and plants
is the fact that many animals are in structure and activities
very much like the himian body; and hence the study of
animals and plants helps us to understand better the human
body and its life-activities. Moreover, thousands of ani-
mals and plants profoundly affect human life. For example,
they provide all the food-supply for mankind ; many harm-
fully influence human health ; and some, such as pet animals
and ornamental plants, contribute much to the pleasures of
life. Clearly, some knowledge of the science of living things
ought to be of great interest to educated people, because it
applies in so many ways directly or indirectly to hirnian life.
Reco^izing this fact, it is the aim of this book (1) to call
attention to the most important facts and principles to be
learned by the study of some selected animals and plants,
and then (2) to show how biological science applies to every-
day human life. In short, this is an elementary book of
applied biology.
2. Living and Lifeless Things. — All things that we know
through our senses are either living or lifeless. This will be
evident if we attempt to write the names of some common
things, grouping them according as they appear to have
life or not. It is not difficult to decide that air, soil, minerals,
and water belong in the list of lifeless things, and that the
common animals and plants are examples of the living ; but
we are puzzled by such objects as dry seeds, undeveloped
STUDIES OF LIVING AND LIFELESS THINGS 3
eggs of animals, some plants in the winter condition, and
many microscopic animals which show no signs of life when
dry. Are such things living or lifeless? Usually it is not
possible to answer until time and proper conditions have
given an opportunity for changes which suggest life-activities,
such as occur during the germination of seeds or the develop-
ment of eggs into young animals.
3. Organisms, Organic and Inorganic Matter. — Except
in science study, we rarely stop to think of the facts brought
out in the problem above ; but for the purposes of our later
work in science it is important that we stop and make such
a survey as suggested above, and recognize clearly that in
this world of ours there are two kinds of things, — the living
(collectively called animals and plants) and the lifeless {e.g.,
air, soil, water, minerals, etc.). Living things are in science
commonly called organisms, and the substances of which
their bodies are composed, or which they form, is organic
maiter. Lifeless substance which has not been formed by
organisms is called inorganic or mineral matter. All sub-
stances, then, in living and lifeless things are composed of
matter which, as we learn through our senses, exists in many
diflferent forms.
4. The Sciences. — Knowledge regarding the living and
lifeless things of nature is systematically arranged in the
natural sciences, A common division of these sciences is that
into (1) the physical sciences (chemistry, physics, geology,
mineralogy), and (2) the biological sciences, which are con-
cerned with living things. We shall see later that there is
much chemistry and physics used in the study of living
things, and hence it will be made clear that these two sciences
deal not only with lifeless things, but also with all substances
and changes which are found in both living and lifeless
things.
4 INTRODUCTION TO BIOLOGY
CHANGES AND COMPOSITION OF LIFELESS AND
LIVING MATTER*
In our later studies of the activities of living things (an-
imals, plants, and man) we shall need to have in mind some
important facts and principles relating to the composition of
both living and lifeless things and to the changes which oc-
cur in them ; and these are outlined in §§ 5-10.
6. Three States of Matter. — It is most convenient to
use the collective term matter for all substances com-
posing the bodies of lifeless and Uving things. Soil, water,
and air are forms of lifeless matter which are examples of
the three states, solid, liquid, and gaseoiis, in which matter
exists.
Matter in one of these states may be transformed into
either of the other states. Thus water, which is ordinarily
liquid, may be cooled and frozen into ice (the solid state),
or it may be heated and changed into vapor or steam (the
gaseous state). Iron and other common metals, which are
ordinarily solid, may be melted into the liquid statfe and at
an extremely high temperature may even change to a
gaseous state. Liquid air is made by reducing the tempera-
ture to —312° F. by subjecting air to great pressure in
powerful machines.
6. Physical Change. — In all such changes of matter from
one state to another (from solid to liquid, or to the gaseous,
etc.) the same substance continues to exist. Ice is only
solid water, steam is a gaseous state of water, molten iron
cools into solid iron, and sugar and salt will dissolve in water.
In these cases there has been a change in the state of material,
but not a change in composition. Such changes which do
* Students who have previously taken courses in chemistry and physics
should read §§ 5-10 as a review of familiar facts from a new viewpoint.
STUDIES OF LIVING AND LIFELESS THINGS 5
not aflfect the composition of substances are called physical
changes. The next two demonstrations are illustrations.
(D) * Dissolve some common salt in water. The salt evidently
imdergoes a change. (The mixture of the salt and water is called
a solution f and since the salt dissolves, it is said to be soluble in
water.) Evaporate the water. Taste the dry substance left.
Has the salt been permanently changed by being dissolved in
water ?
(D) Mix some powdered sulphur and iron-filings, and then use
a magnet to separate them. Evidently the composition of the sul-
phur and iron was not changed.
That branch of science which treats of the state of matter
and its physical changes produced by heat, light, sound,
electricity, gravitation, etc. was formerly called natural
philosophy, but is now usually known as physics.
7. Chemical Change. — All matter is subject to another
kind of change in which the composition is affected and new
substances are formed. Ordinary burning of wood or gas,
and dissolving baking soda in vinegar or other acid, are com-
mon examples. The substances burned, or dissolved in acids,
♦ Directions for practical work are printed in the smaller type through-
out this book. Problems for individual work in the laboratory are marked
(L). Most of the laboratory work requires the supervision of a teacher, but
it will be found that most exercises require little or no apparatus and may be
conducted in the ordinary class-room, if, as in many schools, there is no special
laboratory.
The letter Z>, at the beginning of a paragraph in small type, indicates
that the practical work suggested is recommended for demonstration by the
teacher ; but when marked Z> or L it may be assigned, if the teacher prefers,
as a problem for the individual work of the students.
All subject-matter in the larger type is suitable for recitations. Inmost
cases the laboratory problems and the demonstrations should be taken in
regular order, because much of the text in larger type interprets and supple-
ments the practical work. In preparing any section for recitation or examina-
tion, students should review the laboratory directions in small type, for
many fundamental facts there given are necessary for the intelligent read-
ing of the text in larger type.
6 INTRODUCTION TO BIOLOGY
are changed to other substances. Such transformations
which aflfect the composition of matter are chemical changes.
(D) Bum a small piece of magnesium wire by holding it in the
flame of a gas- or an alcohol-lamp. Is the i)owder left after burn-
ing the same substance and in the same form as the magnesium wire
that you burned ? (If magnesium wire is not at hand, a very nar-
row strip of zinc, or fine iron-filings, may be substituted.)
(D) Mix powdered sulphur and iron-filings, and heat (in a test-
tube or on an iron spoon). Compare with the result of the experi-
ment in i 6. The heat causes the iron and sulphur to combine,
forming sulphide of iron.
8. Elements. — ^The nature of chemical change will be
clear after some further consideration of the composition of
matter. In chemistry^ the science which treats of the com-
position and chemical changes of substances, we leam that
all forms of matter — all living and lifeless substances in
land, air, and water — are composed of about 80 elements,
of which about 20 are very conunon. Some of these ele-
ments exist naturally in the solid state ; for example, iron,
copper, lead, sulphur, gold, nickel, silver, platinum, car-
bon, magnesium, aluminum, tin, and zinc. Some others,
like mercury (quicksilver), are liquid; and still others are
gases, of which the two known as oxygen and nitrogen con-
stitute the greater part of the air.
Chemical Symbols, — For convenience in writing the names
of the elements, chemists have adopted certain symbols or
abbreviations. The ones which will be most needed for
reference in this book are : H for hydrogen ; N, nitrogen ;
0, oxygen ; C, carbon ; S, sulphur ; P, phosphorus ; Na,
sodium (or natrium) ; K, potassium (or kalium) ; Fe, iron
(or ferrum) ; Ca, calcium (lime). A table giving the
full list may be found in any elementary textbook of
chemistry.
STUDIES OF LIVING AND LIFELESS THINGS 7
9. Compounds of Elements. — Now, the elements have
the power of combining with each other so as to form various
compounds. To illustrate : the burning of magnesium is a
chemical combination between magnesium and oxygen of
the air — two elements are here united to form a new sub-
stance, which is a compound of magnesium and oxygen and
is known as oxide of magnesium. Such a union of any
substance with oxygen is called oxidation. The burning of
coal is in part a combination between the elements oxygen
and carbon, but coal contains elements besides carbon. All
ordinary burning or combustion is an oxidation; that is,
the forming of a combination between oxygen of the air and
some other elements. In all cases of chemical change there
is a combining of elements into new or different substances.
Such substances composed of two or more elements are
called compounds. Most of the materials in the solid matter
of the earth are compounds ; water, which is the most
abundant substance, is a compound of the elements hydro-
gen and oxygen ; and most of the materials in the bodies of
animals and plants are compounds. Air, however, is not a
chemical compound ; the nitrogen and oxygen are not united
into a new substance, but are simply mixed together, just as
dry sand and sugar can be mixed without change of com-
position in either. Other minor constituents of the air will
be mentioned later.
Using the chemical symbols for the elements, chemists
write MgO for magnesium oxide formed by burning mag-
nesium (Mg) in the oxygen of the air, ZnO for zinc oxide
formed by burning zinc in the same way, and FeS for the
compound formed by heating a mixture of iron and sulphur
(§ 8). The formula MgO expresses the composition of a
molecule of magnesium oxide and means that it is made up
of one atom of magnesimn united with one of oxygen. This
8 INTRODUCTION TO BIOLOGY
is an example of the simplest possible eompomid. In most
compomids of two or more elements there is a greater pro-
portion of certain elements, and this is expressed by nmn-
bers after the elements of which there is more than one
atom in the eompomid. Examples are: water, written
H2O, meaning that Jiwo atoms of hydrogen (H) are combined .
with one of oxygen (O) ; and sulphuric acid, written H2SO4,
meaning that two atoms of hydrogen, one of sulphur, and
four of oxygen are combined to form the acid.
In the above examples of forming compounds of mag-
nesium and oxygen, iron and sulphur, there are two ele-
ments; but more than two elements are often involved in a
chemical change. One compoimd of several elements may
cause a chemical change in another compoimd.
(D) Ordinary baking soda is a compoimd of four elements —
carbon, hydrogen, oxygen, and sodium (HNaCOa). Put a piece of
soda as large as a pea into a test-tube and pour on it some drops of
vinegar, which also is composed of several elements. What happens ?
Does the liquid in the tube taste like baking soda or vinegar ? Heat
slowly and evaporate to dryness. Taste powder left in tube. Has
some new substance been formed ? In this case the elements of two
compounds united to form two new compounds — the gas (carbon
dioxide) which escaped into the air and the solid substance (sodium
acetate) left after evaporating the water.
This dissolving of baking soda in acid illustrates the
most conunon kind of chemical change, for most substances
on the earth are compounds. It has been noted that the
burning of magnesimn involves only two elements, but the
ordinary burning of coal and other common fuels is a union
of compounds with oxygen, resulting in several new com-
pounds in the smoke and ashes.
Disintegration of Compounds. — Not only may elements
unite to form compounds, but these may be separated into
simpler ones or even into their constituent elements. For
STUDIES OF LIVING AND LIFELESS THINGS 9
example, water may be formed by bmiiing hydrogen gas so
that oxygen of the air unites- with the hydrogen as shown by
the formula H2O, but water may also be decomposed by an
electric current passed through it in a suitable apparatus
and the two constituent elements (hydrogen and oxygen) in
gaseous form be collected separately. Such disintegration
of compounds into the constituent elements is not so com-
mon in nature as is change of compounds into simpler ones.
10. Composition and Changes of Living Matter. — The
principles stated in §§ 5-9 have been illustrated by lifeless
matter, but we shall see in later lessons that they are also
applicable to living matter. Plants and animals are com-
posed of (10 to 15) common elements united in very com-
plex substances; and in their bodies there are constant
physical and chemical changes connected with all life-activi-
ties. Especially do foods eaten and oxygen from the breathed
air become involved in numerous changes in the living matter
of animal and plant bodies. Many times in later lessons we
shall need to refer to the principles of chemistry and physics
which §§ 5-9 review.
Summary : (1) Both living and lifeless matter undergo
physical and chemical changes ; and (2) all matter is com-
posed of elements, usually combined in compounds, and
capable under certain conditions of new recombinations into
other compounds.
CHAPTER II
THE CHARACTERISTICS OF LIVING THINGS
CHEMICAL COMPOSITION OF LIVING COMPARED
WITH LIFELESS THINGS
In order to understand the relations of living and lifeless
things, we need to know whether the organic matter of
living things is composed of peculiar substances which are
not found in inorganic matter. A few simple experiments
will show some remarkable similarity of composition.
11. Water in Living Things. — Water, which is itself life-
less, forms a large part of the bodies of animals and plants,
•as the following experiments show.*
(D) Cut leaves or pieces of stem from a green plant (preferably
a succulent one). Slices of potato, apple, or tomato will make
interesting comparison with the pieces of leaves and stem. Weigh
the plant pieces with a good laboratory scale or a simple balance,
place in a warm, dry place (e.g., over a radiator, stove, lamp, or in
sunlight). When very dry, weigh again. The loss in weight repre-
sents approximately the amount of water evaporated. What per-
centage of the original weight represented water? Keep the dry
pieces of plant for the experiment in § 12.
(D) Carefully weigh some small bits of fresh meat. It is advis-
able to use an amount equal to the fresh plant materials in the pre-
ceding experiment. After drying thoroughly, weigh again, as was
done with the plant materials in the first experiment. What per-
centage of the animal substance was water ? Compare with the plant.
* In preparing for recitations and examination, students should review
the laboratory work in small type and the illustrations, as well as the main
text in larger type.
10
THE CHABACTERISTIC8 OF LIVING THINGS 11
These experiments give us a general idea of the large
amount of water in animal and plant bodies. It requires
very careful experimenting with delicate balances for weigh-
ing, and special apparatus for drying, in order to determine
the exact amount of water in plant and animal matter.
Careful investigations by chemists have shown that the body
of a higher animal (e.gf., a dog) is nearly 70 per cent water.
This is believed to be true also of the human body. This
water is derived from that which we drink and also from
foods. Thus potatoes contain about 78 per cent water,
milk 85 per cent, tomatoes over 90 per cent, apples over 80
per cent, and lean meat over ^0 per cent.
It is evident that water must play an important part in
the life of animals and plants ; and in this connection it is
interesting to note the abundance of water and its wide dis-
tribution over the earth. All good modem textbooks of
geography emphasize the close relation between the distri-
bution pf water and that of animal, plant, and himian life.
Why are there so few Hving things in deserts?
12. Gaseous substances, which are themselves lifeless,
may be obtained from the bodies of animals and plants.
(D) Pack the plant material left after drying in the experiment
in § 11 in the bowl of a clay tobacco-pipe, close the mouth of the
bowl with soft clay or plaster of Paris, support the pipe by a wire
or by a retort-stand, heat the bowl in the flame of a gas- or alcohol-
burner until it reddens and smoke (gases) begins to issue from the
pipestem, then light the smoke with a match. One may heat some
splinters of wood or sawdust and burn the gas in the same way. A
piece of dried meat heated in the same way gives off gases, which
may be burned. It is also possible to bum the gases from dry or-
ganic matter heated in a slender test-tube, or in one of the hard
tubes known as ignition tubes.
Such experiments prove that combustible gases may be
obtained from animal and plant substances after the water
12 INTRODUCTION TO BIOLOGY
has been removed. Chemists have shown that these gases
are not present as such in the animals and plants, but the
elements of which they are composed are united in chemical
compounds which change to gas when heated. This will be
clear when we recall that coal or oil (both of organic origin)
when heated in retorts at the gas-factory, give oflf illimiinat-
ing gas. Gas as such is not present in the solid coal, but it
has compounds whose elements recombine when heated and
form the gas. So it is in the bodies of animals and plants ;
the elements necessary to form the gases are present, and
the gases are produced by heating, which causes new recom-
binations of elements.
13. Carbon is a prominent part of the solid substance of
animals and plants. After the gases were given oflf in the
preceding experiments, a black substance was left in the pipe.
The same thing occurs when we bum an ordinary match ;
that is, certain gases burn and produce the flame and a
charred mass is left. This black material we know as char-
coal ; at the gas-factory a similar substance from heated coal
is called coke. The black color of coke and charcoal is'
due to the presence of an element known to chemists as
carbon. The easiest way to get pure carbon is by charring
white sugar with intense heat. Coke and charcoal contain
other elements besides carbon. Chemists know more than
one hundred thousand compounds of carbon with other ele-
ments. It is especially abundant in the organic compounds
of which the bodies of animals and plants are composed,
and consequently must be also in the food from which the
body substances are made.
14. Mineral substances enter into the composition of ani-
mals and plants. If we reheat the charcoal obtained from
either the animal or the plant matter in the preceding demon-
stration, by holding it with a wire in the hot flame of a gas-
THE CHARACTERISTICS OF LIVING THINGS 13
or alcohol-bumer, the carbon which it contains will soon
burn ; that is, combine with oxygen to form carbon dioxide
(CO2), and only mineral matter or ashes will remain. An-
other example is the familiar fact that when wood (or any
plant matter) is burned there is formed a mass of red-hot
coals or embers, which if allowed to cool quickly, becomes
charcoal ; but if the coals remain heated, they soon change to
ashes, because the carbon is burned. Obviously, the char-
coal obtained by heating ordinary animal or plant substances
is a mixture of combustible carbon and incombustible mineral
matter or ashes. Charcoal made by heating sugar leaves
no ashes, because it is pure carbon, which in burning com-
bines with oxygen and forms carbon dioxide.
A chemist could prove by careful analysis that the ashes
from either plant or animal substances are a mixture of
several compounds containing the elements calciimi, sulphur,
iron, potassium, sodium, and other elements. These are all
found in the soils, and in the water of lakes, rivers, and seas.
15. Summarizing our inquiry into the composition of
living things, we have found that water, carbon, gases, and
mineral matters may be obtained by analysis of living
things. All these substances are also found in lifeless or in-
organic matter. It is evident then that chemical composi-
tion alone does not enable us to distinguish between living
and lifeless matter. And yet in most cases we have no diffi-
culty in deciding whether a certain thing is dead or aUve.
How do we know? The answer will be found in the life-
activities.
LIFE-ACTIVITIES
16. Distinguishing between Living and Lifeless. — The
conclusion reached in the preceding lesson was that all the
materials entering into the composition of living things
14 INTRODUCTION TO BIOLOGY
(animals and plants) are also found in inorganic things,
such as soil, water, and air; and hence animals and plants
cannot be distinguished from lifeless things by the sub-
stances entering into their composition. What, then, does
distinguish the living from the lifeless? Let us first try to
answer this question by examining a livi^ animal {e.g., an
insect or a frog) and, if possible, determine just how it is
different in behavior from lifeless objects, such as a stone or
a dead animal.
Life-ActiviHes of an Animal
17. A living animal moves automatically or spontaneously.
By this we mean that within the animal there is machinery
for producing motion. A stone or any other lifeless thing can
move only through the action of some external agency; it
may fall (gravitation), or be moved by swiftly flowing water,
by a violent wind, or by some animal. Not only do most
animals have the power of locomotion, but also there are
internal movements, such as breathing and the beating of
the heart, which go on continuously as long as they live.
The statement that a living animal has machinery for
producing motion reminds us of the steam-engine, but a
little study shows that the engine is not automatic, as is the
body of a living animal. For example, an engine requires
the attendance of an engineer to supply it with water and
fuel (that is, to feed it) ; but a frog is able to obtain its own
food (fuel) and water to feed itself. Careful study of all
machines invented by man shows that the movement
which at first may appear to be as automatic as that of an
animal is really dependent upon regular human attendance.
Among common objects there is nothing lifeless which seems
to have automatic movement, and in the vast majority of
cases it is easy to decide that an animal is dead if there are
no evidences of movement.
THE CHABACTEBISTICS OF LIVING THINGS 15
18. A living animal requires food if it is to continue to
live and move and grow. This is a fact so well known that
it needs no demonstration. There is nothing like animal
growth among lifeless things. If a saturated solution be
made by dissolving as much common alum (or copper sul-
phate) as possible in boiling water, and then a stone be
dropped into the alum solution, the stone will be coated
by alum crystals, but the stone will not increase. If a lump
of alum be dropped into the saturated solution, the mass
will increase by the addition of more alum crystals to the
surface. This illustrates the fact that stones, crystals,
minerals, and other inorganic things cannot add other sub-
stances to themselves and make them really a part of their
own bodies. On the other hand, living animals can live and
grow on food derived from other animals or from plants.
For example, a frog may eat smaller frogs ; but it may also
eat earthworms or plants, and the substance of these will be
changed and built into that of the frog. This power of a
living animal to take food unlike itself and to make it over
into its own substance is known as assimilation (meaning to
make like or identical).
19. A living animal breathes j but a lifeless object does not.
If we watch a frog, or a higher animal, we see muscular
movements concerned with pimiping air into and out of
the lungs; and we call this process breathing. In many
simple animals there are no lungs, but there are several ways
of proving that they require air, and that they change the
air when they breathe it. That land animals require air is
shown by the fact that they die very quickly if placed in
a jar from which the air has been pumped out with an air-
pump, and aquatic animals will soon die in water from
which the air has been withdrawn by an air-pump or ex-
pelled by prolonged boiling. The following experiment wiU
16 IN;rRODUCTION TO BIOLOGY
show a simple method of testing the changes produced in air
which has been breathed : —
(D) Pour some lime-water or barium-water into a small bottle
and blow air from the human lungs through a small glass tube (or
straw) into the water. What happens? The change in the lime-
water is due to a gas, called carbon dioxide, much of which is added
to the air while it is in the lungs.
The same gas is formed by ordinary burning. Wrap a piece
of wire around a small candle, light it, and then lower it into a tall,
wide-mouthed bottle. After a time the flame will flicker out.
Then lift out the candle and put in a little lime-water. Compare
the lime-water with that changed by air from human lungs.
That carbon dioxide is in the air in a room or outdoors may be
shown by exposing fresh lime-water in a tumbler until a white
film forms on the surface of the water.
Take a tall, wide-mouthed bottle or fruit-jar, wash and rinse
with fresh water, then lower into the jar a frog inclosed in a loose bag
of cheesecloth or mosquito-netting with a string attached so that
the frog may be lifted out quickly without inverting the jar or leav-
ing it uncovered more than for a moment. One of the preceding
experiments suggests that human breathing changes the air of rooms ;
and hence it is important that the jar be held at an open window
where fresh air may enter while the frog is being placed in the jar.
A good way is to keep the jar full of water until the moment when
it is emptied at an open window and the frog placed in it. The
water will prevent the jar becoming filled with the air of the school-
room. A second jar, treated exactly like the first, but without a
frog, should be kept beside the first for comparison. Leave the frog
in the jar, carefully covered, for two hours, quickly lift it out, poiu*
in 10 cc. of lime-water, replace cover, and shake. Did any change
occur in the lime-water when it came into contact with the air which
the frog had been breathing? Test the air in the second jar which
has no frog.
The above experiments prove that some change occurs in
air when animals breathe it. Until a later lesson we need
not take time to consider just what this change is. For
our present purpose it is sufficient to have shown that ani-
THE CHARACTERISTICS OF LIVING THINGS 17
mals do change air when they breathe it, and that by means
of lime-water we can demonstrate breathing of animals in
which we cannot see breathing movements.
20. Living animals have the power of refyroducing new
animals like themselves. Frogs' eggs gradually develop into
new frogs, and insects' eggs into insects of the species which
produced the eggs. We shall later study the development
of some animals from eggs, but for our present lesson it is
sufficient to note that books on embryology (the science of
development) state that all higher animals develop from
eggs, which are small masses of living matter separated from
the parent animals. No lifeless object has any such power
of separating from itself a small body which is able to take
food and grow into a body like the original one. The power
of reproduction is, then, characteristic of living animals.
21. All animals after a time lose the power of moving,
breathing, etc. ; that is, they die. We are familiar with the
fact that individuals of a given kind or species live for a cer-
tain length of time and then grow old and die. For ex-
ample, an elephant has been said to live 200 years, a horse
40, lion 35, cat 40, toad 40, sea-anemone 50, crayfish 20,
vulture 118, eagle 100, pike and carp 200, squirrel and mouse
6, pig 20, sheep 15, fox 14, and hare 10 years. However, it
is certain that most individuals of these species live a much
shorter life ; for example, relatively few horses live over 20
years. The life of all individual animals, even though they
escape accidents and disease, is of limited duration. They
are like clocks wound up to run a certain period of time.
Ultimately the machinery of life stops and the animal bodies
rapidly decompose into substances which show no signs of
ever having been alive.
22. Summarizing, we have seen that a living animal has
the following activities : it moves ; it breathes ; it takes
18 INTRODUCTION TO BIOLOGY
food for assimilation; it reproduces. It has still other
peculiar powers which will be considered later. None of
these is found in lifeless objects, such as stones or dead ani-
mals. All such processes — moving, eating, breathing, re-
producing — which are peculiar to living animals, are
known as life-activities or functions. These are not per-
manent in any individual animal, for after a certain length
of life the individual animal loses its life-activities — we
commonly say that it dies — and the lifeless body soon
changes to the condition of inorganic substances which make
up air, water, and the soil of the earth.
Life-Activities of a Plant
To one who has never studied botany it may seem that
most of the activities named above as characteristic of
living animals are absent from living plants ; but a careful
examination shows that living plants move, breathe, require
food, and reproduce, and in still other ways resemble animals
in their life-activities.
23. Living plants have movement. It is true that most
plants with which we are familiar are not capable of loco-
motion (i.e., movement from
place to place) ; but the same is
true of many lower animals. On
the other hand, there are many
lower plants which have loco-
motion like that of some lower
Fia. 1. The " sensitive plant " animals.
&^foTd?d^^te'i'Sr^^^^ Locomotion in animals is only
{After Detmer.) One phase of their movements;
and much more impressive are
the constant movements of internal organs, such as the
heart and the lungs. There are many similar cases of plants
THE CHARACTERISTICS OF LIVING THINGS
19
Fig. 2. Leaf of Venus
fly-trap adapted for
catching insects. (From
Strasburger.)
able to move certain organs, e.g., the Mimosa (" sensitive
plant '0 moves its leaves and branches when touched
(Fig. 1) ; the Oxalis, the bean (Fig.
13), and certain clovers fold their
leaflets at night ; the Venus fly-trap
(Fig. 2) has peculiar leaves able to
snap together and catch insects;
many plants twine their stems around
supports (Fig. 3) ; and plants bend
toward the light when growing near
a window. All such cases show that
animals have no monopoly of movement. In addition to
such plants which have locomotion and others which move
leaves and other parts, there are in plants movements which
can be detected only with the aid of a
' good microscope.
(D) Examine any plants available which
show any of the movements mentioned above.
Many will be found at greenhouses. Leaf-
lets of the Elodea (an aquatic plant) are
excellent for the movements visible with the
microscope. A piece of an oyster's gill wiU
show microscopic movement going on in
animals.
24. Plants require food if they are to
continue to live and grow. It is a well-
known fact that ordinary garden plants
will not grow well unless there is in the
soil a supply of fertilizer (a kind of crude
plant food). It will be shown, in a later
lesson, that green plants do not grow well if kept long
in darkness, because light enables them to make in their
leaves such foods as sugar and starch.
Fig. 3. The lower
leaf-stalk (petiole)
has twined around
the stem of another
plant.
20 INTRODUCTION TO BIOLOGY
26. Plants breathe. In no plant is it possible to see
breathing movements, as in some giyiifnAls ; but it is possible
to prove by the lime-water test that changes are produced
in the air by the breathing of plants.
(D) Take about twenty pea or com seedlings (directions for
raising such seedlings are given in § 81 of the "Applied Biology").
Place in a loose bag of netting, such as was used for the experiment
with the frog (§ 19). Then lower the seedlings into a wide-mouthed
jar filled with fresh air, and after 24 to 48 hours lift out the seedlings
and test the air by pouring in some lime-water. A second jar with-
out seedlings, but otherwise treated inexactly the same way, should
be kept for comparison (control exjieriment). Take all the care
suggested for the corresponding experiment with the frog. Compare
results with that experiment. The breathing in the plants is so
much slower than in animals that the same amount of change in the
lime-water is not to be exx)ected.
A potted plant, such as a geranium or a begonia, if placed under a
bell-jar beside a small dish filled with lime-water and kept in the
dark, will give proof that the plant causes changes in air, just as ani-
mals do when breathing.
In trying such experiments with seedlings or with any full-grown
plants which have green color, keep the jar covered so as to exclude
light. The reason for this precaution will be clear in a later lesson
which deals with the effect of light upon plants with green color.
Other experiments connected with later lessons will give
further pi-oof that plants breathe, and will help to explain
how they breathe.
26. Plants have the power of reproduction. We are fa-
miliar with the reproduction of many conmion plants from
seeds and of others from cuttings or slips (e,g,, geranium,
coleus, begonia). Many plants (ferns, mosses, .horsetails,
mushrooms, etc.) form small bodies, known as spores, from
which new plants grow. And many of the microscopic plants
reproduce by automatically dividing their bodies into two equal
parts, and these half-size plants soon grow to the full size.
THE CHARACTERISTICS OF LIVING THINGS 21
27. As in the case of animals, individual plants ultimately
die, but some plants may live to a very great age. Some of
the giant sequoias of California, the largest of which are
nearly 300 feet high and more than 100 feet in circumfer-
ence, are probably at least two thousand years old, and
some botanists estimate that they are over four thousand
years old. Some of the famous oaks of Europe are believed
to be eighteen hundred years old, but our largest American
oaks are probably less than five hundred. There are in
Europe specimens of chestnut, olive, cypress, yew, and other
trees which are probably much more than one thousand
years old. It is impossible to estimate accurately even after
the trees are cut down and the so-called *' annual " rings of
the trunk counted; but some of these trees were famous
four or five hundred years ago, and they are certainly of far
greater age than that which any animals are known to have
reached.
28. Characteristic Life- Activities of Animals and Plants.
— Our brief studies of a living animal and a living plant
have shown us the following striking points of resemblance :
(1) movement, (2) need of food, (3) growth, deriving the
new substance from food, (4) breathing, (5) reproducing.
All these activities or changes are found in living plants and
animals and none of them in lifeless bodies. These are life-
activities characteristic of living things.
We see, then, that in order to distinguish accurately be-
tween living and lifeless things we must determine whether
there are evidences of life-activities. Every year dealers
in seeds, gardeners, farmers, and the scientists at the gov-
ernment laboratories must test samples of seeds in order to
determine whether they are living or lifeless. Chemical
analysis will not settle such a question; and so the only
way to test seeds is to put them under conditions where some
22 INTRODUCTION TO BIOLOGY
or all of the life-activities may be manifested. In short, the
seeds must be planted under conditions favorable for growth.
Likewise, dormant animals must be carefully examined for
evidences of life. For example, small animals of certain
species which are often abundant in soil where pools of
water have dried up in midsummer may appear perfectly
dead when viewed with the microscope ; but they begin to
move, eat, and manifest other life-activities soon after they
are placed in water.
29. The Machinery of Life-Activities. — We have seen
that certain activities in animals and plants make the living
things different from the Ufeless ; and in Chapters III and V
we begin the study of the structure and working of the living
machinery which in the animal or the plant moves, takes
food, breathes, grows, and reproduces. In order to under-
stand the working of any complicated machinery, we must
first take it to pieces and examine its structure, and later
find out the use and work of each part. To this end we shall
examine with considerable care the structure of certain ani-
mals and plants in the lessons that follow.
CHAPTER m
FIRST LESSONS IN PLANT BIOLOGY*
30. Plants. — The word " plant '* usually calls to mind
familiar trees, garden vegetables, and ornamental plants
(" flowers ")• In fact, most of the plants known to those
who have not studied botany belong to the highest groups,
which are often called the flowering plants. But the king-
dom of plants is not limited to the flowering plants (also
called seed-plants) ; for in addition it includes a vast variety
of forms known in popular science as flowerless plants. Ex-
amples of these latter are ferns, mosses, toadstools and
mushrooms, sea-weeds, molds, yeasts, and bacteria. Some
of these are of microscopic size, and all are in general appear-
ance quite different from the familiar flowering plants ; but
in many important respects there is much similarity. It is
therefore possible by careful study of some common flower-
ing plant to learn many interesting and useful facts that are
conunon to plants in general. The common bean plant is
excellent for this beginning study. It can be conveniently
grown from seed planted in gardens or in boxes in the school-
room from six to eight weeks before needed for study.
The following account of the structure and life of a plant
is based on the bean plant, but it will serve as a general guide
for the study of any flowering plant.
* To Teachers : Chapter V, on insects, may precede these plant lessons,
if it is desired to study insects in the early autumn. However, this and the
following chapter need materials which are not so easily kept as are the
necessary insects.
23
24 INTRODUCTION TO BIOLOGY
STRUCTURE OF A FLOWERING PLANT
31. Plant Organs. — If we examine any of the most com-
mon flowering plants, such as a bean plant, we notice first
that it has several parts. These are roots, stem with its
branches, buds, leaves, flowers, and sometimes fruits with
seeds. These parts are organs. An organ of a plant or of
an animal is a structure for doing a particular work; for
example, lungs are organs for breathing. The roots, stem,
leaves, and flowers are the plant's organs for taking food, for
breathing, for reproducing — in short, for carrying on all its
life-activities or functions. How can the organs of the plant
perform these functions? In order to answer such a ques-
tion, even in part, it is necessary that we study the general
structure of the roots, stem, buds, leaves, flowers, and fruit.
The Roots
32. General Structure of Roots. — (L) Carefully dig up a young
plant, and note how firmly the roots anchor it in the soil, and how
the particles of soil cling to the small rootlets. Note that there is
no definite boundary between root and stem.
That all the parts of a plant below the surface of the soil
do not belong to the roots, as is popularly believed, is evi-
dent if we compare the roots of a bean plant grown from a
seed planted four inches deep with those on a plant from a
seed placed one inch deep in the soil. In both plants the
roots are grouped at about the same place, and the deeper
planted one has more than three inches of stem below the
surface of the soil, but at first showing no roots. After
growing six or eight weeks, other roots usually start from the
stem nearer the soil surface. Stems of many kinds of plants
will form roots if they are kept in moist soil.
The main root is called primary root, and the branches
secondary roots.
FIRST LESSON 8 IN PLANT BIOLOGY 25
On roots of some plants (clover, beans, peas, etc.) there
may be seen small spherical bodies, some of them as large
as one-eighth of an inch in diameter (Fig. 4). These are
rooUtvbercles. In the later lessons on bacteria (microscopic
plants), these tubercles will be referred to
as caused by bacteria which are useful as
makers of a peculiar kind of plant food sup-
plementary to that furnished by manures or
fertilizers in the soil.
There are on roots, especially on the
younger branches, many delicate rooUhmrSj
which are important absorbers of water from
the soil. They are usually attached to the
soil particles and are broken when plants are ^^^: ^' '^"^^^
•^ ^ cles on root of
dug up and the soil washed from the roots, bean.
(Why are gardeners careful not to disturb
soil around roots when transplanting delicate plants? Why
are paper pots and egg-shells often used for growing young
plants that are to be transplanted?)
Seedlings of radish, oats, clover, barley, and other plants,
grown on moist blotting paper, are excellent for study of root-
hairs.
(L) Examine a root and note that a layer of rind or bark (cortex,
or cortical layer) is easily scraped or stripped from the central cylin-
der of wood. Test the strength of this central cylinder of a small
root by pulling and by bending. How are strong roots of advantage
to the plant ? Tear a root lengthwise and note the fibers that make
it strong.
33. Microscopic Examination of Roots.* — (D or L) Mount on an
object-slide, with cover-glass, some small roots from Tradescantia
stems that have been standing in water, or from seedlings of bean,
* To Teachers : This and later microscopic studies of plants may be
abbreviated to leave only as much as can be seen with a hand-lens. How-
ever, at least one microscope is desirable for demonstrations.
26
INTROBUCTIOK TO BIOLOGY
radish, clover, oats, or barley, grown on moist blotting-paper.
Using first the low power of the microscope, note(l) a cap-
like structure on the tip of the root (Fig. 5), (2) a somewhat
opaque central cylinder (Fig. 6), (3) a transparent rind out-
side, (4) the outermost layer of the rind is the very transparent
epidermis.
Cells. — Now, using the high power of the microscope, it is easy
to see that the root is composed of cells arranged as shown in Fig.
Fia. 5. Longitudinal sec-
tion of tip of barley root,
showing root-cap which
covers the rounded end
of the root.
Fig. 6. Cross section of root, showing
central woody cylinder; rind or cortex:
root-hairs with particles of adhering
soil. {After Frank.)
5. The boundaries of the cells, called cell-walls^ are prominent;
but within the walls may be seen a granular, semi-transparent sub-
stance, which is the living matter or protoplasm of the cell. Some
dead cells with only the cell-walls remaining may be seen, especially
in the central and older part of the root at some distance from the
tip. In the central part of a root the walls of its constituent cells
become hard and woody. All dry wood from roots or stems is com-
posed chiefly of the walls of dead cells.
Growth of roots, and indeed of all parts of plants, is due chiefly
to multiplication of the cells by division.
34. The Work of Roots. — The two primary functions of
roots are (1) anchoring plants in the soil and (2) absorbing
water in which are dissolved certain substances needed for
growth of plants. Water enters the cells of roots chiefly
FIRST LESSONS IN PLANT BIOLOGY
27
through the root-hairs. Certain roots (e.gf., parsnip, carrot,
beet) store large amounts of food for the planVs use in the
next growing season. The binding together of soil particles
by roots is very important in preventing great erosion during
heavy rains.
The Stem
36. General Structure of a Stem. — (L) The places where leaves
are attached are joints or nodes of the stem, and the parts of the
stem between the nodes are
internodes. Compared with
the hard and woody stems of
shrubs and trees, the stem of
the bean plant and of numer-
ous common plants is soft
{herbaceous) ; and soon after
the formation of the pods
(fruit) the plant begins to
wither and then it dies.
Hence the duration of life of
a bean plant is one growing
season; and such a plant
which does not naturally live
over winter is an annual.
Name five other annuals.
With a sharp knife, peel off
the rind from the stem of a
full-grown bean plant. Be-
neath the rind is the hard
woody cylinder which con-
stitutes the main body of the stem. For convenience we shall
apply the popular term wood to the part of the stem inside the bark,
i.e., to the woody cylinder, remembering that in some stems there is
soft tissue called pith especially abundant in the center of the stem.
The wood is very soft in the young parts of stems, and resembles
ordinary wood only when old and hardened. For example, the stem
of a bean plant two weeks old is soft and brittle, but after six or eight
weeks the oldest part of the stem becomes so woody that when peeled
Fig. 7. Diagram of transverse section
of one-year stem, c, cambium; &, a
line marking boundary between central
wood and the bark ; e, epidermis ; p,
pith. Inside the cambium is a ring of
groups of wood tubes (shown as small
black rings). {After SchmeU.)
28 INTRODUCTION TO BIOLOGY
it closely resembles steins of wiDow and other trees that are properly
considered wood.*
Between the rind and the wood of the bean stem is a soft
slippery substance or tissue which makes it easy to peel off the
rind from the wood. The same kind of soft tissue may be found
between the bark and wood of trees (willow twigs are excellent for
examination).
In the center of the soft substance between the rind or bark and
the wood of stems whose structure is similar 'to the bean stem, there
is a layer of cells which are active in growth and division. This is
the growing layer or cambium. On its inner side new wood cells are
formed and added to the older wood of the stem, and on its outer
side the new cells formed are added to the inner surface of the bark.
Hence growth and division of cambium cells add new cells to both
wood and bark, resulting in increased diameter of the stem.
On the outside of rind like that of the bean stem is a thin layer,
the epidermis. Part of the rind is green in color, which is due to a
substance most common in leaves and known as leaf-green or cMo-
rophyll.
With a sharp knife or section-razor, make a cross cut {transverse
section) of a bean stem. Examine with hand-lens, and note relative
thickness of rind and wood. In the center of the wood is a softer
substance, known as pith; and in an old bean stem there is a cavity
in the pith.
(Optional.) Split open lengthwise (longitudinal section) a stem
from an old bean plant, and note the extent of the pith and of the
central or pith cavity. Tear the wood apart lengthwise, and note
that it is " stringy." Pay special attention to the arrangement of
the wood, bark, and pith in a longitudinal section of a stem where
♦To Teachers: In applying the term "wood" to the woody cylinder
of the stem, it is evident that wood is not synonymous with xylem. That
which is popularly called "wood" includes the medullary rays, which are
parenchyma from the standpoint of histology. Hence wood in the popular
sense — and it is a popular and not a scientific word — is a combination of
xylem and parenchyma, except any very soft central parenchyma which
is commonly recognized as pith. For the purposes of elementary plant
study the histological distinctions between the tissues of the woody cylinder
of stems are of little significance, and so the word ''wood" is used in this
text as it is used by all the world except some writers who try to make it a
synonym for xylem.
FIRST LE8S0N8 IN PLANT BIOLOGY
29
it branches. Make diagrams showing position of rind, cambium,
wood, and pith.
36. Microscopic Study of Stem. — (D) With a razor or very
sharp knife cut a very thin transverse section of a bean stem, mount
in water on a glass object-slide. Examine with low power the rind
and its outermost layer (epidermis), the wood, and the pith. Use
a higher power of the microscope, and observe the cells. In some of
the cells the living matter or protoplasm is visible, but some cell-
walls in the central
part of the stem are
older and contain no
living matter.
In the outer part
of the wood of the
stem as seen in a
transverse section
are some large open-
ings, which appear
to be empty cells.
These (see Fig. 7)
are cut-off tubes,
the wood-tubes or
wood-ducts, in some
of which water as-
cends the stem from
the roots. The
smaller and thick-walled cells surrounding the wood-tubes are hollow
fibers (wood-fibers). Bend a bean stem from which the rind has
been peeled, and notice its strength. Bend a very young piece of
stem and notice that it breaks easily. The explanation of the
difference between the two stems is that the tubes and fibers are not
fully formed and hardened in the young stem. This is of practical
importance in cultivating plants, for growing tips and young stems
require more delicate handling than do the older and harder ones.
In Fig. 8, of a longitudinal section, it is shown that the woody
fibers are long cells, and that some of the wood-tubes have peculiar
spiral and ring thickenings of their walls, while others have numerous
thin spots (called pits).
Notice in Fig. 7 and in a transverse section that the wood-tubes
are in groups. In annual stems Uke the bean and in the first year of
Fig. 8. Longitudinal section of fibro-vascular
bundle of sunflower stem, c, cambium; d, wood-
ducts; b, bast cells or fibers; p, pith or paren-
chyma cells; «, sieve-tubes; w, wood cells be-
tween ducts. {After Wettatein.)
30
INTRODUCTION TO BIOLOGY
our common trees and shrubs * the groups of wood-tubes are arranged
in a single ring in the outer woody part of the stem and just beneath
the bark (Fig. 9). In the inner part of the bark and opposite the
groups of wood-tubes are other very small tubes and fibers. The
cambium lies between the tubes and fibers of the bark and those
in the wood. The tubes of the bark are called sieve-'tuhes (because
of sieve-like partitions at intervals in the tubes) ; and they serve
Fia. 9. Photomicrograph of half of a transverse section of clematis stem.
Six fibro-vascular bundles shown, c, cambium, which was accidentally
split in the three bundles on left side of the picture. Center of stem
filled with pith (p) cells, which extend outwards between the bundles.
Note the wood-ducts in the inner part of each bundle. (Photograph by
E. F. Bigelow.)
to conduct sap down the stem. Groups of the bark fibers (often
called bast fibers) are the ** strings " seen when the bark is shredded
into strips.
In some young stems, such as those of dutchman's pipe, the tubes
and fibers on the two sides of the cambium are massed so as to make
groups like the six shown in Fig. 9. These groups are called fihro-
vascular bundles. Notice Fig. 9 carefully and observe that the cam-
bium (C) marks each bundle into an outer or bark part and inner
woody part.
* All plants of the groups dicotyledons and conifers.
FIRST LESSONS IN PLANT BIOLOGY
31
In many common stems, the tubes and fibers are not so definitely
massed in separate groups or bundles, but they form a more or less
complete ring as shown in Fig. 10. But even in such stems it is
possible to recognize as a fibro-vascular bundle the group of tubes
and fibers inclosed within the
oval ring in Fig. 10. Every
such group is equivalent to
one of the six bundles shown
in Fig. 9.
(D) Exhibit a transverse
section of a first-year stem
of Aristolochia (dutchman's
pipe) or clematis, compare
with Fig. 9.
( D or L) The location and
the work of the wood-tubes
may be shown as follows:
Cut off a fresh bean or similar
stem, and stand its lower end
in red ink (eosin solution) for
a half-hour. Carefully strip off the bark and notice the position of
the red color in the wood. Make transverse cuts in several places
and examine with a hand-lens. Also, split longitudinally the wood
of a stem into which red ink has ascended, and examine.
Fig. 10. Diagram of one-fourth of a
transverse section of a sunflower stem.
r, rind; c, cambium; w, wood-ducts.
The ring marks the equivalent of one
bundle in Fig. 9. (After Frank.)
To sum up statements made above, water from the roots
goes up the stem in the wood-tubes that are just within the
cambium, and fluids go down in the tubes of the bark (sieve-
tubes) that are just outside the cambium. Thus the *' circu-
lation " or movement of fluids up and down the stem is
confined to the inner part of the bark and the outer part of
the wood.
37. Two-Year and Older Stems. — All the foregoing re-
lates to stems whose structure is like that of the bean and
which live less than a year and also to the first-year stems
of our common trees and shrubs which live a number of
years.
32 INTRODUCTION TO BIOLOGY
Transverse sections of two-year, three-year, and older
stems from ash, oak, chestnut, and many other trees show
more than one ring of large wood-tubes, usually a ring for
each year of growth (" annual rings "). These rings of wood
are produced by the cambium whose cells grow and divide so
as to add new cells both to the wood and to the bark. Thus
the woody part of the stem is continually increasing in diame-
ter and the bark would become thicker each year if it did not
scale off on the outside almost as fast as new bark is formed
by the cambium.
Many stems, such as those of oak and beech, have promi-
nent lines radiating from near the center. These are the medul-
lary rays, sometimes called pith-rays because composed of
compressed masses of pith cells. These rays will be studied
later in connection with the economic uses of woods (§ 300).
In old trees the tubes in the dead heart-wood do not carry
water. This explains why the centers of trees may be
rotted or burned out, and the trees
continue to live as long as a layer of
wood just beneath the bark remains
healthy.
38. Another Type of Stems. — The
stems of com, lilies, grasses, and
many other plants of the groups
known as monocotyledons (§ 44)
have a structure different from that
_ ^ described in the foregoing which
Fig. 11. Transverse sec- , ^ ^ ^, . , n , ,.
tion of corn stem showing relates to the pliants called dicoty-
rind (r), pith (p), and ledons and also to the conifers. The
numerous fibro- vascular . !.«. . ,
bundles (»). main difference is that in stems like
that of corn there is no cambium
layer, and the fibro- vascular bundles are scattered irregularly
in the stem (Figs. 11 and 12). .
FIRST LESSONS IN PLANT BIOLOGY
33
Structure of Corn Stalk. — (L) Cut a dry corn stalk (stem)
transversely. The smooth rind is on the outside, and inside the rind
is the soft pith throughout which fibro-vascular bundles are scattered
irregularly.
Split a stalk lengthwise, and the fibro-vascular bundles appear as
long threads which are easily pulled out. Trace these long threads
through nodes. Do they all continue upward, or do some of them
turn toward the rind at the
nodes? Try to trace some of
the bundles out into the bases
of leaves left on the nodes.
(D) Stand a piece of green
corn stem for a short time in red
ink, and then cut both trans-
versely and longitudinally in
several places, and note where the
colored liquid ascends the stem.
Each fibro-vascular bundle in
the corn stem has an upward
movement of fluids (air and
water from soil) in the large
tubes in the part of the bundle
directed towards the center of
the stem, and a downward move-
ment in the small sieve-tubes in
the part of the bundle towards
the rind (see Fig. 12).
(Optional.) Examine with a
microscope a thin cross section
cut from a green stem, or from
one preserved in alcohol or formalin solution. Observe that all parts
of the section are composed of cells most of which are thin-walled pith-
cells. Scattered irregularly among these are the fibro-vascular bundles.
See the '* Applied Biology, " § 161, if fuller notes are wanted.
39. General Functions of Stems. — The primary work of
ordinary stems is (1) support of the leaves in positions
adapted to their work (§ 43), (2) conduction of fluids between
roots and leaves, and (3) breathing.
Fig. 12. Transverse section of a
fibro-vascular bundle in corn stem.
Lower part of the figure is towards
center of stem, a, d, sp, ducts or
wood-tubes ; sir, strengthening
fibers; st, sieve-tubes ; wp, woody
pith. (From Osierhout.)
34 INTRODUCTION TO BIOLOGY
(1) In the work of support, stems make use of the elasticity
and rigidity of woody tissues and also of turgidity due to
water in the cells. A very young plant depends largely
upon turgidity, for if water is withheld from the soil, the
leaves and stems become wilted. A young Windsor-bean
plant growing in a pot will illustrate this. If the soil be al-
lowed to dry, the stem will lose its turgidity (become wilted)
and will droop. If then water is added to the soil, the stem
will become turgid and rigid with remarkable rapidity, and
within two or three hours will be as erect as ever. Stems
of older plants of most species do not bend over when water
is wanting, for woody tissues furnish the necessary rigidity.
A straw of wheat or rye is a splendid example of rigidity due
to woody tissues arranged in cylindrical form.
(2) The work of conducting materials is, as we have seen
in the bean plant, due largely to the tubes in wood and bark
which are specially fitted to conducting liquids up and down
the stem. Other cells of the stem also play a part in con-
duction of liquids, especially in the transverse or horizontal
direction.
(3) Breathing by stems occurs through certain openings
in the outside layers of tissue. The epidermis of herbaceous
stems has stomata similar to those of leaves. Examine with
microscope epidermis from such plants as bean, tradescantia,
and begonia. In plants with corky bark the stomata are re-
placed by openings known as hnticles (meaning lens-shaped)
or stem-pores. Good examples may be seen on young stems
of cherry and plum. Notice that on the older parts of these
stems the lenticles are elongated by growth. The holes in
cork used for bottles look like holes made by worms, but
they are lenticles. Examine a bottle cork and note the
parallel holes which originally extended through the thick
corky bark, thus allowing air to penetrate to the active cells.
FIRST LE880N8 IN PLANT BIOLOGY 35
In addition to the three functions named above, many
stems have special work such as climbing, food storage, and
even serving as leaves. Some of these are referred to in this
book in §§ 294-307, and they are more fully treated in
§§ 169-178 in the " Applied Biology."
The Buds
40. Position and Kinds of Buds. — At various places on
a bean plant are buds, which will unfold later. Some of these
buds will form flowers, and hence are called flower-buds;
others will unfold as leaves and are therefore leaf-buds. At
the ends of the main stem and its branches are terminal buds
which, by growth, lengthen the stem and branches. If the
terminal bud of the main or a branch stem be destroyed,
lengthening of the stem will cease. For this reason gar-
deners often break off the terminal buds of climbing or pole
beans (e.g., limas) when they have reached a height of about
six feet. This has the effect of sending into the developing
fruits (pods) much of the nutriment which would have gone
to make the useless extension of the stem.
(L) Examine huds on a young bean plant, (1) with reference to
their position, and (2) separate carefully the parts of both a leaf-
and a flower-bud.
Twigs and their buds (§ 294, § 295) may be studied at this time.
The Leaves
(L) Students who have not previously examined leaves should
give some attention to the general form of such simple leaves as
apple or beech and maple. Notice bladcy petiole, veins, and stipules
at base of petiole.
41. Leaf and Leaflets of Bean Plant. — (L) Bean leaves appear to
be arranged in groups of three (Fig. 13) ; but they are really three parts*
or divisions of one leaf. Such a leaf is called compound, and the three
divisions are called leaflets. The supporting leaf-stalk {petiole) is
36
INTRODUCTION TO BIOLOGY
Fig. 13. a, leaf of bean with three
leaflets on petiole ; b, position at
night. (From Detmer.)^
attached at a node of the stem and can be easily disting^uished from
a branch of the stem, because the petiole has an enlargied and some-
what flexible attachment to the stem ; and also because a longitu-
dinal cut through the node shows
that the leaf is attached chiefly
to the rind, while a branch is
attached firmly to the wood of
the stem. Each of the leaflets
is attached by a thickened joint
similar to that of the petiole at
the stem. These flexible joints
of the leaf and leaflets allow the
drooping of the leaves at night
and on hot days (Fig. 13 6).
Examine any one of the leaf-
lets, and note that the expanded
smrface {blade) is a thin sheet of tissues supported by ribs and veins.
The central rib is the mid-rib. Hold the leaflet up so that light
may shine through it, and with a hand-lens examine the delicate
veins {veinlets) between the ribs. Make an outline sketch showing
the structure of a complete leaf and the detailed structure of one of
the three leaflets ; label all
the parts named above.
42. Microscopic Struc-
ture of a Leaf. — (D or L)
Any convenient leaf, such
as tradescantia, may be
used for this exercise.
With fine-pointed forceps,
strip off a very small piece
of the thin transparent
epidermis from both the
upper and lower surfaces
of a leaf, and mount for
microscopic study with low power. The epidermal cells of some
leaves are not as irregular as those shown in Fig. 14.
Scattered among these irregular cells are pairs of crescent-shaped
cells set together so as to leave a small pore or opening through the
epidermis. Each pore is a leaf-pore or stoma (a Greek word mean-
ing an opening, plural stomata). It opens into a small cavity (air-
FiG. 14. Epidermis from certain plant
leaves. A, upper surface. B, lower sur-
face with stomata. (From Straaburger .)
FIRST LESSONS IN PLANT BIOLOGY
37
space) beneath the epidermis (Fig. 16). Are the leaf-pores present
and equally abundant on both upper and lower sides of the leaf
examined ?
These two crescent-shaped cells around the pore are called guard-
cells. It has been noticed that when water is abundant in the plant
these cells swell and become more crescentic
in form, leaving a larger opening than when
the leaf is drier. It is believed by most
botanists that the guard-cells are able to
prevent excessive evaporation of water
from the air-spaces, and thus conserve
water when necessary.
Cut off a leaf transversely, and with a
strong hand-lens examine the cut end.
Note the transparent epidermis which covers both sides and the
edges ; that is, entirely surrounds the leaf. Between the upper and
lower epidermis the center of the leaf appears to be filled with a
green-colored, somewhat granular material (middle-tissue or meso-
phyll). Also notice the cut ends of the colorless veins.
Fig. 15. Two stomata ;
one closed, one open.
Fig. 16. Cross section of privet leaf, ep, epidermis; pi, palisade cells in
upper part of middle tissue; A;, crystals; a, air-spaces; v, vascular mid-
rib; 8tt a stoma. (From Sirdshurger.)
If the hand-lens is a strong magnifier, it is possible to see that
the green-colored middle-tissue is more compact toward the upper
surface and appears to have small cavities in the part next the lower
epidermis. This will be very clearly seen when the compound
microscope is used for examining the cut end of a leaf.
38 INTRODUCTION TO BIOLOGY
(D) Examine with a microscope a very thin transverse section
from the end of a leaf cut like the one described above. (To cut
such a section, hold the leaf between two pieces of elder-pith, or slices
of potato, or roll a leaf to form a rather tight cylinder. Keep the
edge wet with water and with a very sharp razor shave off a number
of very thin slices. Float these off the razor into water and with
fine forceps or a small brush transfer these sections to a drop of
water on a glass-slide and place the cover-glass in position.] Com-
pare a transverse section with Fig. 16 and locate all the structures
in the cut end of the leaf that was seen with the hand-lens. Note
that the epidermis is transparent and without chlorophyll, except
in the guard-cells at the leaf-pores. Compare with the epidermis
which you previously stripped off and examined in surface
view.
Most of the cells in the middle-tissue have green bodies (the chlo-
rophyll-bodies, or ohloroplasts). The compact upper part of this
middle-tissue is seen to be composed of elongated cells (palisade cells)
set closely together (Fig. 16). In the lower part of the leaf the cells
are irregular in shape and there are numerous air-spaces. Some of
the.se spaces communicate with the outside through the leaf-pores,
and thus air may enter the leaf and become widely distributed
throughout the air-spaces.
Also, in the middle-tissue examine the cut ends of the leaf
veins. Wood-tubes in them are connected with those of the
petiole which lead to the water tubes or wood-ducts in the wood of
the stem.
43. The Work of Leaves. — There are three important
functions of ordinary leaves : (1) evaporation of water ob-
tained from the roots through the stem ; (2) making sugar
and starch from the elements in carbon dioxide and water,
obtained respectively from air and soil ; and (3) breathing
or exchange of air in the air-spaces of the leaf through the
leaf-pores or stomata. Each of these processes will be
studied more carefully later (§§ 62-72). In addition to these
three primary functions, the leaves of many plants are able
to store food, catch insects for food, to serve as climbing
organs and as protective prickles. Some of these special
FIRST LE880N8 IN PLANT BIOLOGY 39
forms of leaf functions are mentioned in §§ 308, 367 of this
book and more fully in § 183 of the '' Applied Biology."
44. Classification of the Bean Plant. — By this we mean
the relation of the bean plant to other plants. In the first
place, the bean plant is a member of the great division of
flowering plants or seed-plants. Within this division are
many families, one of which is the family of leguminous
plants (including peas, beans, vetches, clovers, locusts, and
numerous other plants which have irregular flowers similar
to those of the bean plant). In this family are included a
number of bean-like plants which are somewhat different
from ordinary garden beans (e,g,, Windsor beans and horse
beans). The common garden beans belong to the geniLs
Phaseohis and to the species mdgaris, hence the scientific
name is Phaseolus vulgaris. The various kinds of ordinary
beans are varieties of the same species. The Windsor and
horse beans belong to another genus.*
The relation of the bean plants to other plants is shown
in the table of classification below. It is one of the Di-
cotyledones in Division IV, the seed-plants.
(There are four divisions or primary groups of plants.)
Division I. Thallophyta (simplest plants).
(1) AlgsB (with chlorophyll). Examples: sea-weeds,
many minute aquatic plants.
(2) Fungi (without chlorophyll). Examples : molds,
mushrooms.
Division II. Bryophyta. Examples : mosses and liverworts.
Division III. Pteridophyta. Examples : ferns, horsetails, lyco-
pods.
* To Teachers : Some teachers prefer to let students acquire gradually*
a conception of classification of plants or animals, while to others it seems
best to take such opportunities as this section affords for presenting to the
class considerations of the meaning of classification such as are given in
Chapter VII of the ''Applied Biology."
40 INTRODUCTION TO BIOLOGY
Division IV. Spermaphyta (or Phanerogamia).
(1) Gymnospennse. Examples: cycads and coni-
fers.
(2) AngiospermsB.
(a) Monocotyledones. Examples: palms, grasses,
lilies, orchids.
(6) Dicotyledones. Examples: most of the
common trees, vegetables, and " flowers."
The plants in Divisions I, II, III are often termed ** Spore-
Plants," " Flowerless Plants," or " Cryptogams " ; while those of IV
are " Seed-Plants " or " Flowering-Plants."
Flowers, Fruits, and Seeds. — These parts of flowering
plants are not needed for the life of the individual plant,
but only for production of new individuals, that is, for per-
petuation of the species. Since in the first part of this course
in biology we are chiefly concerned with the structure and
life of individual animals and plants, it is most convenient
and most interesting to leave the study of flowers, fruits,
and seeds for Chapter XII, which compares the reproduction
of many living things. However, they may be studied at
this time by turning to §§ 335-343.
CHAPTER IV
THE WORK OF THE ORGANS OF A PLANT: AN
INTRODUCTION TO PLANT PHYSIOLOGY
45. Plant Functions. — The preceding lessons have dealt
largely with the bean plant ; but all the facts to which special
attention has been given are true of the vast majority of the
plants which have roots, stems, and leaves. The one plant
has been studied as a type or example of common plants. In
the preceding lessons special attention has been given to
structure, and functions have been mentioned only inci-
dentally. We are now ready for a study of the working of
the various organs of common plants of which the bean plant
has been selected as an example. That division of biology
which deals with the work or functions of the organs of
plants is commonly known as plant physiology.
In order to carry on their life-processes, plants must
have food, water, and oxygen, as all animals do. These
necessary materials must be obtained from the soil and air
by all ordinary land plants. This chapter on the physiol-
ogy of plants is concerned with the work of plants in getting
food, oxygen, and water and in using these materials in their
life-processes. We shall study first the entrance and move-
ments of water in plant roots, stems, and leaves ; and then
in later sections we shall consider how plants get and use
food and oxygen.
46. The Need of Water. — In one of the first experiments
we found that plants contain a large amount of water.
41
42 INTRODUCTION TO BIOLOGY
Moreover, any one who has ever cultivated plants knows
that unless the soil is kept moist the plants will wither and
die. Evidently water must be of great importance ; and so
it will be of interest to study (1) how water gets into the
plant organs (root, stem, and leaf) and (2) what work water
does in these organs.
47. Source of Water. — It is obvious that ordinary plants
which have roots must get most of their water from the soil.
It might be supposed that some water from rain and dew
which wets the leaves is absorbed ; but that this is exceed-
ingly small in amount and insufficient may be proved by
taking a potted plant and covering the soil with waterproof
cloth so that rain and dew may touch the stem and leaves
but not the roots. Under such conditions most kinds of
ordinary plants would soon wither.
48. Water in Soil. — How can plants get water from soil
which appears to be very dry? The answer to this question
is that apparently dry soil is not without water. It is not
necessary, or even desirable, for many plants that the soil
be wet ; that is, contain free water which might be drained
off. On the contrary, it is best for most plants if the soil
contains moisture in the form of thin films adhering to the
particles of the soil. We have already noticed that the root-
hairs adhere to the soil particles (Fig. 6), and therefore they
are in the best possible position for absorbing water from the
soil.*
49. Rise of Water in Soil. — For oiu* plant studies the
most important points concerning soil moisture are as fol-
lows : The water stored in the deeper layers of the soil sup-
* If the students working with this book have not in some previous
science work had lessons on water in soil, some work in this line should be
introduced. Osterhout's "Experiments with Plants," pp. 103-121 ; Burkett,
Stevens and Hill's "Agriculture for Beginners," pp. 10-15, suggest good
experiments.
PLANT PHYSIOLOGY 43
plies moisture to the layers nearer the surface. This is due
to capillary action or capillarity, of which good illustrations
are the rise of oil in an ordinary lamp-wick and of coffee into
a lump of sugar which just touches the liquid. Water is
continually being lost at the surface of the soil, owing to
evaporation and to absorption by plants, and it is as con-
stantly coming up from below. But it may be lost by evapo-
ration from the surface soil more rapidly than it can come
from the deep layers, especially if the surface is hard-packed,
as on a road, or if the surface is covered with large numbers
of plants, such as weeds or grass, which take much water
from the soil.
Mulching, — Excessive loss of water from the surface of
the soil may be prevented by mulching, two methods of
which may be illustrated by reference to potato cultivation.
(1) In the usual way, the soil is cultivated on the surface
in order to kill weeds which use water needed by the growing
potato plants, and in order to keep the surface of the soil
in a dusty condition. This checks the capillary action a
few inches below the surface and moisture comes up to
within reach of. the roots of potato plants, but not to the
surface where it would be wasted by evaporation. Such a
condition is known in agriculture as a dust-mulch. On many
farms that are conducted according to modem scientific
ideas, one may see on hot dry days in the summer the culti-
vating machines at work pulverizing the surface of the soil
in order to keep in the moisture. This is the main secret of
success in the " dry farming " in many western States.
(2) The second kind of mulch is illustrated by the follow-
ing method, often successfully used in growing potatoes in
dry regions and in dry seasons. The potatoes are planted
shallower than usual and the entire field is covered with
several inches of straw. The potato plants grow up through
44 INTRODUCTION TO BIOLOGY
the straw, but most of the ordinary weeds do not. The
straw prevents evaporation of water from the surface, and
in dry weather the soil is found to be moister than soil
treated by the dust-mulch method. In fact, a great objec-
tion to the method is that the soil often becomes too moist
after heavy rains. This method is often used in orchards,
where the weeds and grass are cut and spread over the
ground around the trees, instead of cultivating the soil to
keep in the moisture.
• (3) In arid regions it is necessary to irrigate in order to
grow potatoes. If the water is sprinkled over the surface,
much of it is wasted by evaporation and moreover the soil
surface becomes packed so that there is much loss of water
from the deeper layers of soil. Modem irrigation methods
avoid these objections to surface sprinkling by allowing the
water to flow in furrows about a foot deep and soak into the
soil beneath the surface. When the soil is well soaked, the
furrows are filled with soil, and then the surface is kept in
dust-mulch condition. This method is extensively applied
for conserving water in the irrigated gardens, farms, and
orchards of the western States.
It is obvious from the foregoing account of water in soil
that this is one of the most important problems connected
with growing useful plants, and horticulturists and agricul-
turists have found it important to understand the scientific
facts concerning the water of the soil and its use by plants.
50. Absorption of Water by Roots. — That pressure is
developed when water is absorbed by the roots may be
demonstrated as follows : —
(D) Cut the. stem of a healthy plant near the ground, and attach
a glass tube by means of rubber tubing as shown in Fig. 17 and put
in a small quantity of water. Geranium, dahlia, tomato, sunflower,
corn, many young shrubs and trees, and grape-vines have stems
PLANT PHTSIOLOGT
45
which make it easy to attach the tube. Make marks on the tube,
keep the soil moist, and note the rise of water in the tube for several
days.
Instead of a straight tube, botanists often use an S-shaped
tube, and fill the lower bend of the tube with mercury.
Since mercury is 13.6 times heavier than
water, it is easy to compute the height to
which a colunm of water might be forced.
The root-pressure of a grape plant is suflft-
cient to force a column of water up 10
meters (how many feet?).
51. Work of the Root-Hairs. — How
does the water concerned in root-pressure
get into the root from the soil ? This is
the question which naturally arises in our
minds when we observe the preceding ex-
periment. Scientists have studied the
microscopic structiu*e of the roots of many
plants and have found no openings or
pores through which water can get into
the root, and so they have concluded that
the water must soak through or be
absorbed through the walls of the surface
cells of the root and especially through
the walls of root-hairs. In order to make this method of
absorption clearer we must try some experiments.
62. Osmosis. — ( D) Select a cork that will fit firmly in the mouth
of a ** diffusion shell " (a membrane bag which may be purchased
from dealers in scientific apparatus), bore a small hole in the cork,
and into the hole fit a glass tube of small caliber, and 18 to 40 inches
long. Fill the shell with dark-colored molasses, insert the cork in
.the shell, wrap and tie around the cork a strong coarse thread, and
support with a retort-stand, wooden tripod, or otherwise, so that
the diffusion-shell will hang in water (Fig. 18). Note the movement
FiQ. 17. «, cut-ofiE
stem of a plant;
r, rubber tubing
for attaching glass
tube (t).
46
INTRODUCTION TO BIOLOGY
of the molasses up the tube for several days. Change the water
when it becomes discolored by exuded molasses. If the shell does
not break, or leak at the cork, the column of fluid may rise to a
height of 8 or 10 feet. The glass tube may be
made longer by joining on other tubes with
pieces of rubber tubing for the joints.
A piece of gold-beater's membrane, or of
fish-bladder, may be tied over the large end
of a thistle-tube, which has been filled with
molasses. Support so that the membrane
will be in water.
I
w
The membrane used for the above
experiment has no visible pores. The
entering water must pass through spaces
which are far too small to be seen with
the aid of the strongest microscope.
Such diffusion or absorption of water or
solutions through a membrane without
visible pores is called osmosis (sometimes
osmose) in the science of physics. The
verb to osmose will be used in this book.
For an explanation of osmosis one must
study the advanced books on physics;
but for the purposes of plant study it is
sufficient to remember the above experi-
ment as an illustration of osmosis.
A point needed for our later studies
is that water having certain substances
in solution may osmose. For example,
in the above experiment the water be-
came discolored by the exuded molasses
(solution of sugar in water), proving that some of the
molasses osmosed. Evidently the molasses did not osmose
outward as rapidly as the water osmosed into the molasses,
Fig. 18. Diagram of
apparatus for os-
mosis, t, glass
tumbler; m, mem-
brane bag; w, level
of water in tumbler;
8, sugar solution in
bag and up to level
x; c, cork tied into
mouth of the bag;
g, glass tube of i
inch bore fitted into
hole in cork. Rise
of fluid higher than
X indicates osmosis
of water into the
sugar solution faster
than that of the
sugar solution out
into the water.
PLANT PHYSIOLOGY 47
otherwise the column of fluid would not have been forced
up the tube.
53. Osmosis or Absorption by Roots. — The above prin-
ciple of osmosis applies to the absorption of* water by roots
as follows : The cells of the roots (especially the root-hairs,
Fig. 6) allow osmosis as the membrane of the diffusion-shell
did. Thus water from the soil, containing mineral sub-
stances in solution, osmoses into the root. There is one
difference between this osmosis in the root and that in our
experiment ; namely, that the cells of the root are filled with
substances which attract water but do not themselves osmose
out, as did the molasses. Hence water continually osmoses
into the root, while the chief cell-substances do not osmose
out into the soil. Some substances in root cells do pass into
the soil but in very small quantities.
(D) That living plant cells control outward osmosis of their
substances is illustrated as follows: Cut slices of a red beet root.
Drop some slices into boiling water for a few minutes to kill the
cells. Then put all the slices into cold water. Those heated give
out their coloring substance rapidly to the water.
Recalling the pressure indicated by the height of the
water column in the last two experiments, we note that in
both tjases pressure is the result of osmosis or absorption of
water, in one case into the root by the cell-substance, and in
the other case into the diffusion-shell by molasses. Roots,
then, get the water from the soil by a process called absorp-
tion or osmosis through the delicate walls of the root cells,
especially of the root-hairs.
64. In what part of the Root does Water ascend to the
Stem ? — Our experiment on root-pressure (§ 50) showed
that water passes from the soil through the root into the stem.
It has been stated that water gets into the root through its
surface, p^Jticularly through the root-hairs, which greatly
48 INTRODUCTION TO BIOLOGY
increase the amount of surface cells for absorption. What is
the path of water through the root on its way to the stem?
The following experiment shows that water from the soil is
distributed to cells by the wood-tubes.
(D) Cut off the small end of a slender root of carrot or parsnip,
or of any other plant large enough for cutting sections ; and place
cut end of root in a bottle of red ink (eosin in water). After several
hours cut transversely in several places, and note the position of
the red-colored water in the tubes in the woody part (in fibro-
vascular bundles, § 36) . Some of the ink after a time soaks (osmoses)
into the cells in the bark and pith of the root.
66. Why Water ascends Stems. — There are many evi-
dences that water is continually ascending the stems of plants
to the leaves. For example, if we cut off a stem, the leaves
soon wither and dry up, while similar leaves with the natural
connection to the root through the stem remain fresh and
well supplied with water. We have already studied the rise
of water from the soil into the root and from the root into
the stem. What causes the water to ascend the stems of
plants? This is still one of the imsolved problems of botany.
Root-pressure, as we have seen, is enormous. In a young
birch sapling it has been foimd to be great enough to raise a
column of water 18 meters (how many feet?). But root-
pressure alone is not enough to force water to the top of tall
trees, and it does not move water upward fast enough to
make good the loss by evaporation from the leaves on a hot
day. There must be other factors in the elevation of water.
Another possible explanation is suggested by a simple ex-
periment described on page 93 of the '* Applied Biology,"
and illustrated by Fig. 19. A plant is cut off above the
root and the stem inserted and sealed into a glass tube,
which is filled with water and placed upright with its lower
end dipping into mercury. When water is evaporated from
PLANT PHTSIOLOGT
49
the leaves, the mercury rises in the tube (due to air-pressure,
same as " lifting " of water by a '* suction " pump). If
mercury is lifted one inch, it indicates a force able to lift
water 13.6 inches. In this way it has been determined that
some shoots will lift water several meters. Evidently there
is great lifting power or suction force
caused by evaporation from the shoot.
That the " lifting " power is due merely
to evaporation and not to some force
peculiar to living plants may be demon-
strated by tying a bladder or other mem-
brane over a thistle-tube filled with water
and suspended vertically so as to dip into
a cup with oil or mercury. As rapidly as
the water evaporates through the mem-
brane, the oil or mercury rises in the tube.
The apparent " suction " is the same as
in a pimip, and may be explained by the
statement that atmospheric pressure on
the mercury or oil forces it up the tube
as rapidly as the water evaporates. The
same explanation applies to the evapora-
tion of water from the leaves in the pre-
ceding experiment.
Root-pressure, capillarity, and evaporation from the leaves
are the best suggestions concerning the rise of water up the
stems of plants ; but botanists admit that even these work-
ing together do not explain how a plant can lift water as
high as do the highest trees. This is only one of many
hundreds of things in science which it has not been possible
to explain satisfactorily; and in the attempt to get more
knowledge himdreds of scientific men are constantly trying
experiments in new ways.
Fig. 19. Mercury
" lifted " by evapo-
ration from leaves,
w, mercury; c, cork
in end of glass
tube and perfo-
rated to receive
stem. (FromStras-
burger.)
60 INTRODUCTION TO BIOLOGY
56. In What Part of a Stem does Water Ascend? — Does
water go up in all regions (bark, wood, and pith) of the stem?
We can best answer this question by an experiment.
(D) Cut off a shoot (stem with leaves) of a nasturtium, balsam,
com, bean, or other plant, and place the cut end of the stem in a
small bottle with red ink (solution of eosin in water). Watch the
rise of the ink along the stem into the veins of the leaves. Now
cut' across the stem in various places and note that the red color is
in the woody part. Cut a thin section and note that the red color
is in the wood-tubes of the fibro-vascular bundles, which were men-
tioned in § 36.
If some stems be left in the ink for several hours, the sections will
show that the ink osmoses from the wood-tubes into the bark and
pith.
From such experiments as those above we conclude that
the path of water up the stem is through the fibro-vascular
bundles in the woody part of the stem. It was through
these tubes that the water passed in our experiment on root-
pressure (see § 50) and in that on evaporation (§ 55). From
the wood-tubes water slowly osmoses into the cells of the
bark and pith.
57. Water in Leaves and Evaporation. — In the last ex-
periment the red-colored water ascended the stem and passed
along the veins of the leaves. This was possible because the
veins are bundles of tubes directly connected with the wood-
tubes in the stem.
(D) Take a leaf from the bean or other shoot which was used in
§ 56. Scrape the petiole and veins so as to show the bundles of
tubes colored by the red ink. Note (especially good in the leaves of
bean and celery) the connection of the bundles of tubes in the veins
of the leaf with the bundles of the stem ; this is easily done by care-
fully scraping away the surface tissue from one side of the stem and
petiole until the colored bundles of tubes are uncovered.
It is evident from the arrangement of the tubes and the
path taken by the ink that water can pass directly from the
PLANT PHYSIOLOGY 51
wood-tubes of the stem into those in the veins of the leaf, and
thence into the numerous veinlets. In this way water com-
ing up the stem from the root is distributed throughout the
leaf, which is thin and greatly expanded so as to expose as
much surface as possible to sunlight and air, and thus pro-
mote rapid evaporation. It is obvious that one purpose of
the veins of the leaf is to distribute or spread the water so
that it may be evaporated rapidly. That the arrangement
is very efficient is indicated by the fact that the leaves of a
simflower plant six feet high have been proved to evaporate
a liter (how many pints?) of water per day ; and it was once
carefully estimated that a large birch tree with about 200,000
leaves gave oflf 500 liters of water (2^ barrels) on a dry, hot
day and probably averaged 60 to 70 liters per day diuing
the summer.
Estimating Loss of Water from a Plant, — (D) Take a potted
geranium, or other convenient plant, surround the pot and cover
the soil with a sheet of rubber, tin-foil, or waterproof cloth, and
tie carefully around the stem of the plant. Or set the pot in a
battery-jar and cover the top with sheet rubber tied tightly around
both jar and stem of plant. Thus only the upper stem and leaves
will be exposed for evaposation. Now place the potted plant on
a small platform scale (preferably one with two equal platforms
which balance each other), and add the weights until there is an
exact balance. Note from day to day the loss of weight from the
plant. The tube of a glass funnel may be inserted through the water-
proof covering into the soil, and a weighed amount of water poured
in daily to replace the amount lost by evaporation.
In order to make evaporation more rapid than slow dry-
ing from the surface cells of the leaf, the leaf-pores (stomata)
are sometimes opened, allowing watery vapor to escape from
air-spaces in the leaf (Fig. 16). These cavities are sur-
rounded by cells from which evaporation of water takes
place rapidly. Thus the giving off of water is more rapid
when the leaf-pores are open, and less when they are closed.
62 INTRODUCTION TO BIOLOGY
The leaves of many kinds of plants seem to have a very
complete control of the amount of water evaporated, because
in these species the covering cells (epidennis) of the leaves are
thick and sometimes coated with waxy substances or hairs,
and in still other ways are imfavorable for the evaporation
of water from the surface of the leaf. In such plants evapo-
ration appears to take place chiefly through the leaf-pores.
When we remember that an ordinary cabbage leaf has ten
million and a sunflower leaf thirteen million leaf-pores, we
can easily understand how the opening and closing of these
pores may control the amount of evaporation.
The process of evaporation of water from the leaves of
plants is in botany commonly called transpiration, and we
say that leaves transpire, meaning that they give off water
by evaporation. For all practical purposes the words
evaporation and transpiration are equivalent as applied to
the work of leaves. '
58. Soil-Water and Sap. — Throughout this lesson we
have referred to water as ascending in the plant, but it is
never pure water. Water absorbed from the soil always con-
tains some mineral materials in solution. Also, as the water
passes through the plant it absorbs or dissolves other sub-
stances and becomes sap. However, a large part of the
water which goes directly up the stem in the wood-tubes is
not very different from the water of the soil ; and travelers
in tropical countries often cut off pertain vines and drink
the water which runs from the stems.
One other point should be emphasized ; namely, that in
the evaporation of water there are left behind in the plant,
especially in the leaves, all the substances brought in solu-
tion from the soil. Leaves which fall in the autumn give
when burned much more ashes than do those which fall in
the early summer; and the explanation is that evaporation
PLANT PHYSIOLOGY 53
during the long summer season has left behind in the leaf
various materials carried up in solution in water from the
soil, and not needed by the plant.
59. Use of Water in the Plant. — So far in our study of
water, its movement through root, stem, and leaves, and
its final loss through evaporation, we have considered the
purely mechanical processes without reference to the use of
the water while passing through the plant. Now, plants ara
not elevating water simply for the sake of evaporating it
from the leaves ; on the contrary, evaporation is necessary
to make place for more fresh water and so keep up the cur-
rent from root to leaves. This is necessary for the following
reasons : (1) The water carries up in solution indispensable
food-materials obtained from the soil. (2) Water is neces-
sary in many parts of the plant in order to give turgidity
and rigidity. The wilting of plants when the soil is very
dry is due to lack of water in cells and consequent loss of
turgidity. A Windsor-bean plant growing in a pot is excel-
lent for showing this. Allow the soil to dry imtil the plant
wilts, then water the soil. (3) Water is necessary in order to
dissolve sugar (e.g., maple sugar and other sweet juices of
plants) and other food-substances which must often be
transported in solution from one part of a plant to another.
(4) Water is needed because it is used in the manufacture
of such food-substances as starch, sugar, and oils by the
plant. (5) Water in large quantities is needed by growing
plants because such a large proportion of the new substance
is water. (6) Evaporation of water results in cooling the
plant, thus preventing a dangerous amount of internal heat.
In addition to these special reasons why plants need
water, we must remember that all living things require
water (§ 11). Without water there is no life, so far as we
know. Even seeds that are apparently dry contain a cer-
54 INTRODUCTION TO BIOLOGY
tain amount of water (8 to 15 per cent of their weight).
Why are plants and animals not abundant in arid regions?
60. Food of Plants. — In addition to water, which may
be estimated by weighing a plant before and after drying,
there are other substances in plants and these must be
derived from the food.* When chemists analyze the sub-
stances obtained from plants, by the methods used in the ex-
periments of §§ 11-15, they find the following ten chemical
elements : carbon (C), hydrogen (H), oxygen (0), nitrogen
(N), sulphur (S), phosphorus (P), iron (Fe), potassium (K),
calcium (Ca), and magnesium (Mg), in every plant. Still
other elements are found in many plants, but they are not
absolutely necessary for plant life. The first four (C, H, 0, N)
form the chief part of the combustible matter of all plants.
These ten elements which are always found in plants must
also be in their food, and in the next three sections we shall
consider how plants get the food which will furnish these nec-
essary elements for making plant structure.
61. Food-Materials from the Soil. — It is a matter of
common observation that the growth of plants is largely
influenced by the nature of the soil. Every farmer and
gardener learns through experience to distinguish between
barren and fertile soils, and that the addition of manures
and various chemical " fertilizers " increases the growth of
plants. The relation of plant growth to the materials avail-
able in the soil may be well illustrated by the following
experiment.
(D) Growing Plants without Soil. — This may be done by ger-
minating seeds of oats, beans, peas, lupines, and other common
* To Teachers : If Chapter II has not been studied, it is worth while
to turn back and take up §§ 11-15 before considering the food of plants.
If the students are unfamiliar with chemical elements and combinations,
refer at this time to §§ 5-10.
PLANT PHYSIOLOGY 55
plants on moist sawdust, cotton, sand, crushed stone, or other
materials into which roots can penetrate, but which contain no plant
food. When the seeds are well germinated, begin to moisten the saw-
dust- or cotton daily with water in which have been dissolved some
chemical tablets containing the materials such as are found in
good garden soil ; that is, the necessary elements (§ 60). If the
roots are kept moist with such a solution of chemicals, some plants
will develop flowers and seeds. By trying various chemicals in
such experiments, it has been possible for botanists to prove that
only certain elements are necessary in the soil for plant growth.
Most of the elements are common in agricultural soils, but com-
mercial fertilizers rich in nitrogen, potassium, calcium, and phos-
phorus are needed on many farms. The tablets may be purchased
from the Agassiz Association, Sound Beach, Conn., for ten cents a
box, postpaid.
Other interesting experiments in the same line may be performed
by growing plants in different kinds of soils in pots, adding to the
soils various kinds of plant foods and fertilizers sold for garden use
(see catalogues of seed-dealers).
62. Food of Plants without Chlorophyll. ^ — We can better
understand how a bean plant or other green plant gets its
food if we first study the nutrition of plants like the mush-
rooms and the Indian pipe, which have no chlorophyll. Such
plants get their food-materials, containing the ten essential
elements (§ 60), entirely from the soil in the form of (1) cer-
tain inorganic or mineral substances which are commonly
found dissolved in water in good soils, and (2) organic ma-
terials absorbed from decomposing matter, such as leaf -mold,
on which mushrooms and the Indian pipe commonly grow.
The water obtained from the soil by mushrooms contains
compounds with the element nitrogen, and also in various
combinations are other elements {e.g., calcium, phosphorus,
sulphur, potassium, magnesium, iron) which chemists find
in analyzing such plants. These necessary elements may
come in part from the decaying organic materials in the
soil.
66 INTRODUCTION TO BIOLOGY
Now, only plants with chlorophyll can make sugar and
starch,* and so mushrooms and similar plants must get
such foods from some preexisting plant which had chloro-
phyll ; that is, they must live on leaf-mold or other decaying
plant matter from which they can absorb such food. These
carbohydrate foods which are absorbed by the cells of the
mushroom are used by its protoplasm in its life-activities,
especially in making new cell-materials into which the nitro-
gen and the other necessary elements named above are also
combined.
Saprophytes and Parasites, — Plants that get their car-
bohydrate foods from decaying organic matter are often
called saprophytes (from Greek words for " rotten " and
*' plant ")• Many plants without chlorophyll are sapro-
phytes. Some absorb the necessary food from living plants
or animals, and such plants are parasites (e.g., dodder, and
mistletoe to some extent).
63. Food of Plants with Chlorophyll. — The common
green plants have in their bodies at least the same ten essen-
tial elements as plants without chlorophyll. These elements,
except carbon and some oxygen, are obtained from the soil,
the hydrogen and oxygen in the form of water (H2O), and the
other seven in solution in water absorbed by the roots.
The one fundamental difference between the nutrition of
plants with and without chlorophyll is that the chlorophyll is
a special substance with the aid of which the protoplasm of
leaf-cells is able under the action of light to make the carbo-
hydrate food (sugar and starch) needed by the plant, while
non-parasitic plants without chlorophyll must absorb such
food from the decaying bodies of other plants or animals.
* Sugar and starch are carbohydrates, a term which means a compound
containing carbon (C) and water (H»0), and therefore composed of the
necessary elements carbon, hydrogen, and oxygen (C, H, O).
PLANT PHTaiOLOGT 57
Cells containing chlorophyll are able to make carbohy-
drate food in the form of starch or sugar, obtaining the neces-
sary elements from water, from the soil, and from carbon
dioxide of the air.* Thi& production of sugar and starch
takes place chiefly in the leaves. Probably sugar is first
formed and then changed into starch; but since starch is
usually demonstrable in green leaves exposed to light, we
shall give special attention to its formation. However, it
makes no difference to the plant whether sugar or starch is
formed in the leaves, for they are of equal value as food for
all plant cells.
The stomata allow watery vapor to escape, in transpira-
tion, and they are important also in allowing air with carbon
dioxide to enter the spaces of the leaves, whence the carbon
dioxide passes into the cells with chlorophyll (Fig. 16).
Water reaches these cells by osmosing from near-by veinlets,
which in turn receive water from the roots through the stem.
64. Photosynthesis. — That light is necessary for making
carbohydrates (sugar and starch), the following experiment
shows. Since the process is a synthesis or a combining
depending upon the action of light, it is commonly termed
photosynthesis (meaning combining by light). Briefly, it is
sugar-making or starch-making in green leaves exposed to
light.
Test Jor Starch. — (D) Put a small quantity of cornstarch in a
tube with water, add a few drops of iodine solution (crystals of I in
70 per cent alcohol). Note the blue eolor. Test effect of iodine on
powdered sugar in water. If we had time, we might test in the same
way all the various substances found in plants and animals; but
* The atmosphere contains on the average about three parts of carbon
dioxide in 10,000 of air, measured by volume ; and yet from this exceedingly
small amount of carbon dioxide the green plants get the necessary carbon
for making all carbohydrates, which compose a large part of the solid matter
of plants.
58
INTRODUCTION TO BIOLOGY
we must accept the records of science that no one has yet found any
other substance in organisms which with iodine solution gives this
peculiar blue color seen in starqh. Hence we use iodine as a test
for the presence of starch in animal and plant substances. Some-
times iodine is mixed with chloral hydrate in order to make it stain
more intensely.
Starch Formed in Leaves in Light. — (D) Take two potted plants
(bean, nasturtium, or other convenient plants), leave in a perfectly
dark room (or box) several days, and some
morning take one of the plants from the dark
and set in sunlight. Near the close of the day
take leaves from both plants, dip into boiling
water for a minute, and place in bottles (labeled
** dark," " light ") containing strong alcohol.
When the class meets again, note that the green
color (chlorophyll) has been extracted by the
alcohol (" bleached "). Take a leaf from each
bottle, rinse in water, and place in a beaker or
a small saucer containing some iodine solution
(or better, use saturated solution of chloral
hydrate mixed with enough iodine solution to
color). In which leaf does the iodine test show
presence of starch ? Remembering that the
two leaves have been treated exactly alike
except that one came from a plant left all day
in the sunlight, while the other remained in the
dark, state your conclusion as to the importance
of sunlight in starch-formation.
Another way .of showing the effect of light on starch-formation
is as follows : Take a piece of sheet cork, or thick pasteboard, or
tin-foil, about two inches square, and cut out a triangle, star, or any
figure preferred, from the center. By means of pins or paper clamps
fasten this on the upper side of a leaf of a potted plant, and a piece
of black cloth or sheet of cork on the lower side ; or cut the opening
desired in a strip of tin-foil and then fold this around the middle of
the leaf. , Set the plant in bright sunlight with this leaf supported
so that the upper side will get full illumination. Near the close of
the day cut off the leaf, dip into boiling water, then place in alco-
hol for a day or two. Finally, test for starch with^the iodine (or
chloral hydrate and iodine) as in above experiment. Make a sketch
FiQ. 20. The darker
ends of this leaf
were colored by
iodine, indicating
starch. The center
had been protected
from light.
PLANT PHYSIOLOGY 59
of the leaf, and by colored pencil or shading indicate where starch
was formed. Label the portions of the leaf ** exposed to light '*
and " in dark."
Variegated Leaves. — Many varieties of plants grown in
greenhouses have " variegated '' leaves with large white
areas due to the absence of chlorophyll. That starch is not
formed in these areas can be shown by taking a leaf which
has been in sunlight all day and testing for starch as in the
above experiments. Plants grown for a long time in a dark
room lose their chlorophyll (e,g.y sprouting potatoes in a
cellar), and after a day in sunlight their leaves show no
starch. Why are the inner leaves of heads of lettuce and
cabbage not green ? How is celery bleached or '* blanched " ?
Intensity of Light Required. — It must not be inferred from
the preceding that light must necessarily be in the form of
bright and direct sunlight. Carbohydrate-formation is most
rapid in sunlight, but probably goes on in light of all inten-
sities, even in moonlight. This explains why some species
of plants can live and grow slowly in shaded spots in the
woods ; they make use of the diffuse and weak light which
reaches, them. Moreover, Hght from other sources than the
sun will serve for photosynthesis; for example, electric
lights are sometimes used in greenhouses. The blooming of
Easter hlies may be hastened from four to ten days; and
lettuce grown within fifty or sixty feet of a 2000-candle-
power arc-light, used regularly half of the night, will be ready
for market a week or more before plants not so treated. In-
candescent electric- and gas-lights have a similar effect, but
in lesser degree.
Another illustration of the effect of light is in the rapid
maturing of plants under the influence of the intense light
of the short arctic summer. Also, plants in greenhouses in
winter do not grow as rapidly in a given number of hours of
60 INTRODUCTION TO BIOLOGY
sunshine as they do in summer when the light is so much
more intense. And this is so even when the interior of a
greenhouse is constantly at the average summer heat.
66. Disappearance of Starch from Leaves. — Starch formed
in leaves exposed to sunlight soon disappears, as the follow-
ing experiment shows : —
(D) Take leaves from any plant which has been standing in sun-
light, and place them in alcohol (§ 64) for testing with iodine later to
make sure that starch is present. Then set the plant in a dark room
or box for two nights, take other leaves before light reaches them,
and soak in alcohol before applying iodine. There will be no blue
color with iodine solution, thus indicating that starch has not re-
mained in the leaves kept in darkness.
Where does starch go when it disappears from leaves kept
in darkness? There are two answers : —
First, some of the starch has probably been used in the
cells of the leaf, either changing to other cell-substances
composed of the same elements (e.g., sugar), or combining
with the elements brought in water from the soil and form-
ing compounds containing, besides the C, H, of the starch,
some N, S, and P, and perhaps other essential elements.
The compounds thus formed (containing C, H, 0, N, S, P)
are called albuminous substances or proteins; and some of
these may form some new living matter (protoplasm) in the
cells of the leaf.
A second explanation of the fact that the starch disap-
pears from the leaf at night is that it goes down into the
stem or root, or into the developing flowers or fruits. That
this is what occurs in part is proved by the fact that starch
and other foods (e.g., sugar, and oil with the same elements)
are foimd in these parts of the plant, while all experiments
ever tried have failed to show that these substances can be
formed in cells without chlorophyll. The carbohydrate
PLANT PHYSIOLOGY 61
foods needed in all living cells of a plant must be made in
the cells with chlorophyll. Most of these cells are in leaves.
Starch in Cells. — (D) Thin sections of a potato will show the
starch-grains in the cells. Stain some sections with iodine solution
before mounting, or by drawing iodine under the cover-glass with
blotting-paper. Using low power of a microscope, note the form of
the starch-grains.
66. Digestion of Starch to Sugar. — How can such solid
grains get through the cell-walls in going from the leaves
and then into cells in other organs? The answer to this
question is that starch is easily changed or digested to sugar,
which is soluble in water and can osmose from cell to cell.
This change from starch to sugar is caused by a substance
known as diastase, which is present in cells of leaves and
other organs .of plants. This substance may digest starch
at all times during the day and night, but the starch is
formed during sunlight more rapidly than it can be digested.
At night when starch is not being formed the diastase suc-
ceeds in digesting all the starch which was left in the leaf
at the close of daylight. Hence, starch-grains are not found
in leaves in the morning after being in darkness for several
hours ; but sometimes it is possible by chemical analysis to
find in leaf cells sugar into which the starch has been changed
and which either will be used in the leaf or will pass down
the petiole into the stem.
Sugar Test, — (D) Boil a few grapes, raisins, or prunes in a test-
tube with a Uttle water in order to extract some of their sugar.
Pour some of the extract into another tube, add a few drops of
Fehling's solution (a mixtmre of copper sulphate and Rochelle salts,
used by chemists for testing certain kinds of sugar), heat tube in a
flame, and note the red color of the contents. Test some glucose
or corn-sirup. Add Fehhng's solution to a Uttle starch in water;
does the red color appear? Only sugars like glucose give the red
reaction. White granulated sugar from cane and beet does not.
62 INTRODUCTION TO BIOLOGY
Change of Starch to Sugar. — (D) Boil a small quantity of starch
in water in a test-tube, thus making a very thin starch-paste. Put
half of the paste in a second test-tube and add some diastase (ob-
tained by extraction from plant tissues, and sold at drug-stores).
After a half-hour, take some liquid from each tube and apply the
starch and sugar tests. Results? Conclusions?
Osmosis, — (D) Pour the contents of the two test-tubes into two
gold-beater's bags and hang the bags in tumblers or beakers con-
taining some water, or use the osmose-apparatus described in § 398.
After allowing an hour or more for osmosis, pour some water from
each tumbler into test-tubes, and test with a few drops of Fehling*s
solution. Also test some of the water for starch, using iodine.
Does the starch-paste osmose ? Is there sugar in the water having
the starch without diastase? Conclusion?
67. Path of Foods down the Stem. — It has been stated
above that the foods (chiefly sugar) made in the leaf may
go down into the stem or into the other parts of J;he plant con-
nected with the stem. Also, the proteins (compounds of C, H,
O, with N and other elements) which are formed in the cells of
leaves, may go down into the main stem and thence into
roots, flowers, fruits, or new branches. Obviously the foods
in solution cannot go down in the wood-tubes, because in
them there is the upward current of water from the soil.
Botanists are now agreed that the downward current is in
the bark part of the fibro-vascular bundles, and through the
tubes previously described as sieve-tubes (§ 36). The move-
ment is so slow that it cannot be demonstrated with red
ink, as in the case of the wood-tubes (§ 36). However, the
evidence obtained as described in the next two paragraphs
is no less convincing.
Effect of Girdling. — If a branch of a rose bush or other
shrub be girdled (cutting away a ring of bark down to ^he
cambium) and the injured region covered with wax to pre-
vent drying, a thickened band of bark will slowly develop
above the girdle, but never below. Evidently the tissue
PLANT PHYSIOLOGY 63
above the girdle obtains more food for growth, and this must
come down the stem in the sieve-tubes of the bark, for the
wood continues to have the upward current of water from
the soil. A branch will continue to live, but if the main stem
is girdled the plant will die after some months, for the part
of the stem below the girdle as well as the roots cannot get
the necessary foods from the leaves and so they starve
slowly.
Some trees die because an iron wire has been left wrapped
around them imtil the stem has grown so large that the ring
of wire becomes embedded in the bark down to the cam-
biimi, thus cutting off the sieve-tubes of the bark and pre-
venting foods from reaching the roots. Farmers often
girdle trees in early summer in order to prevent them from
shading crops; but on some kinds of trees the leaves will
remain green all summer if only the bark is girdled. In
order to make the leaves of such trees wilt soon after gir-
dling, it is necessary to chop through the bark and deep into
the wood. Only the wood-tubes in the light-colored outer
layers of wood, known to lumbermen as sap-wood, are im-
portant in the ascent of water; and therefore cutting deep
into the sap-wood of a tree will cut off so many wood-tubes
that the leaves cannot get water to supply loss by evapora-
tion, and hence soon wilt. Even the hardiest trees, like the
honey-locust, will ultimately die after the bark is girdled,
because the supply of food, which came down from the
leaves and was stored in the roots before the girdling, is
gradually used up, and no more can get down to the roots.
It is possible for foods to go up a stem. The leaves of a
branch that has a bunch of grapes might be cut off and the
grapes would continue to grow, foods coming up the stem
in the wood-tubes from other branches with leaves. In the
spring before leaves appear on trees and shrubs, foods go up
64 INTRODUCTION TO BIOLOGY
the stem from the cells in root or stem where they were
stored during the winter. If a branch of a tree be cut off
in winter just above a bud, this will usually grow rapidly in
the following spring and form a large branch. Orchardists
make use of this fact when they prune trees. All these cases
prove that the foods must be able to go up the stem. The
upward movement is in the wood-tubes.
Summarizing, foods in solution in water (i.e., sap) may
be transported either up or down the stem, depending upon
where the supply is located and where needed.
68. Use of Foods Transported from Leaves. — The sugar
and other foods made in the leaves may be used in the plant
in two ways, as follows : —
(1) Used by Cells, — We have seen that living plants re-
quire food, and this is true of every living cell. Some of
the food from the leaves is at once used by cells in roots,
stem, flowers, or developing fruits. Part of the food un-
doubtedly goes to make new particles of living matter (pro-
toplasm) to replace that continually being worn out and
made lifeless. It should be kept in mind that a living plant
is like a moving machine in that activity causes wear or
waste, and hence some food must be used for repair or the
entire plant will soon die. In short, some food must be
used continually in making new protoplasm. If more pro-
toplasm is made than is needed for repair of waste, the
result will be growth ; and usually growth means the forma-
tion of new cells. This is especially true at the growing tips
of plant stems. The making of new protoplasm is known as
assimilation, or constructive metabolism,
(2) Stored in Cells, — If the foods received from the leaves
are not needed for immediate repair, they are stored in vari-
ous parts of the plant (stem, roots, leaves, or seeds). There
are various modifications of plants adapted to storing foods ;
PLANT PHYSIOLOGY 65
but for our present purposes it will be sufficient to mention
that carrots, turnips, and sugar-beets are examples of plants
that store large quantities of plant food in their roots ; sugar-
cane and sago-palm store food in their stems ; the head of a
cabbage and an onion are bundles of leaves stored with
food ; and beans, com, and nuts are seeds stored with foodi
These are examples of various plant organs in which food id
stored for the future use of the plants. It has happened
that man and the herbivorous (plant-eating) animals have
found it convenient to appropriate many of these reserves
of plant food for use in their own food-supply.
These reserved foods of the plant are commonly stored as
starch, which is easily seen (with low power of microscope)
in thm sections of various stems and roots, especially in the
late autumn or winter after storage has been going on dur-
ing the entire growing season. The sugar solution derived
from starch in the leaves osmoses into the cells of the root,
stem, or fruit, and is there changed and stored as starch-
grains. When the plant needs this stored starch, the starch-
grains are changed back (digested) again into sugar, which
is able to osmose out of the cell and into other cells.
But all cells do not store starch. Sugar, from which our
ordinary granulated sugar is obtained, is stored in large
quantities in the cells of sugar-beets and in sugar-cane.
Sometimes the sugar that enters cells is changed to oil, as in
nuts; and when the plant needs food elsewhere the oil is
changed (sometimes to sugar again) and osmoses out of the
cells. In still other cases the sugar from the leaves enters
cells and is used in combination with nitrogen and other
elements from the soil to form protein substances.
Enzymes. — All these remarkable changes which take
place when foods are stored in plant cells have long puzzled
chemists. No one yet understands fully how the plant cells
66 INTRODUCTION TO BIOLOGY
are able to make these changes; but it is known beyond
doubt that these changes do occur regularly in the life of
plants, and further it is known that many peculiar substances
called enzymes {e.g., diastase) are present in plant cells, and
that in some way not understood and not yet imitated in
the chemist's laboratory these enzymes can change sugar into
starch or oil, or these back into sugar. A peculiarity of
enzymes is that they can change other substances without
undergoing change themselves, and that a small quantity of
enzyme can produce a large amount of change. The most
familiar example of an enzyme is pepsin, which in the stomach
of animals and humans digests protein foods.
69. The Oxygen-Supply of Plants : Respiration. — It has
been pointed out that plants breathe or respire and have
the same effect upon air that animals have; namely, they
absorb oxygen from the air and give off carbon dioxide (CO2).
The term respiration is in both plants and animals usually
applied to this combined process ; but it is simpler to con-
sider first how oxygen is obtained.
In a plant without chlorophyll (e.g., a mushroom), oxygen
is absorbed at all times of the day and night by the surface
of the plant in contact with the air ; and the gas then diffuses
or osmoses from cell to cell, for all Uving cells must have a
continual supply of oxygen. If some mushrooms be placed
in a closed jar for many hours, it can be demonstrated by
chemical tests that the air in the jar has lost oxygen. This
will be the same no matter whether the jar is kept in light
or in total darkness.
A bean plant kept in darkness in a closed jar will, like the
mushroom, take up oxygen. The same plant kept in a
closed jar in sunlight will give off oxygen. Apparently the
reverse is true of the mushroom or the green plant in dark-
ness; but let us withhold our conclusion until we have re-
PLANT PHYSIOLOGY 67
viewed previous lessons dealing with carbon dioxide in
leaves.
We have noted that carbon dioxide is used rapidly in
making carbohydrates (in photosynthesis) while the plant is
in light ; but in darkness the plant makes no carbohydrates
and consequently uses no cferbon dioxide. Now, in the
combining of the elements of carbon dioxide and of water to
make sugar or starch, the carbon of the carbon dioxide is
used, but the oxygen is not needed and is set free in the cells
of the leaves.* The amount of oxygen thus freed is very
much more than the plant needs for its oxygen-supply ;
that is, it is very much more than the same plant in darkness
would absorb from the air; and the result is that there is
excess oxygen to be given off to the air. The truth is that
the living cells throughout the plant require oxygen both in
light and in darkness ; but while photosynthesis goes on so
much oxygen is freed from carbon dioxide that the plant
cannot use it all and gives off the excess to the air.
Oxygen Liberated by Photosynthesis. — (D) Place some water
plants, such as Elodea, in a glass funnel which is then placed with
tube upwards in a glass battery-jar filled with water. The water
must be deep enough to more than cover the funnel and its tube.
Fill a test-tube with water and, keeping its mouth below the water
in the battery-jar, invert it over the end of the funnel tube. Set
in sunlight. Bubbles of gas (chiefly oxygen) will rise from the plants
and displace the water in the test-tube. The gas may be tested
for oxygen by stoppering the test-tube before lifting from the water,
and then quickly inserting a glowing taper when the stopper is re-
moved.
♦ Readers who have studied chemistry will be interested in the propor-
tions of CO», HaO, and O in starch-making as follows : (6 COs + 5 HaO)* =
(CaHicOe + 6 Oj)*, which means that to every six molecules of carbon dioxide
and five of water there will be one molecule of starch (CaHioOs) and six
molecules of free oxygen. This formula simply gives the proportions ; for
starch is some unknown multiple of CeHioOs.
68
INTRODUCTION TO BIOLOGY
All kinds of plants, as well ks animals, require oxygen
constantly. They may get the necessary amount directly
from the air, or green plants may get it from carbon dioxide
during daylight. There is, then, no real difference between
the breathing of mushrooms, animals, and plants with chloro-
phyll. The increase in oxygen and decrease of CO2 in the
air around green plants during daylight is obviously due to
the independent process of photosyn-
thesis, not to their breathing.
There are good reasons for believing
that roots of plants absorb oxygen
from the air which is abundant in
good soil. In fact, one scientific
reason for cultivating or tilling the
soil is to mix air with the soil par-
ticles. The water of the soil contains
oxygen in solution, just as the water
in a river contains oxygen which fishes
can absorb by their gills. When the
water is taken up into the plant stem
it probably carries along with it some
oxygen, which is absorbed by the cells
with which the water comes into contact.
Also, some of the large tubes of the fibro-vascular bundles
are filled with air, probably taken in chiefly at the leaf-
pores and also at the bark-pores (lenticels), which are slit-
like openings in the bark leading to internal air-spaces.
The following experiment shows how air may enter the leaf
and pass through the stem.
(D) Select a wide-mouthed bottle and a stopper with two holes
(Fig. 21). Take a leaf with a small round petiole and push the
petiole into a hole in the cork and almost to the bottom of the
bottle. Also fit a glass tube into the other hole of the cork. Fill
Fio. 21. Apparatus to
show that air can enter
the leaf. (From Stras-
hurger.)
PLANT PHYSIOLOGY 69
the bottle half full of water. Use vaseline to make the apparatus
air-tight. Apply suction to the glass tube (a small bicycle pump
with the leather on the plunger reversed, so as to " draw " air out,
and connected by rubber tubing with a reversed valve from a bicycle
tire will answer, if an air-pump is not at hand). Air will exude from
the cut end of the petiole and appear as bubbles in the water. The
experiment may be reversed by attaching the petiole to rubber
tubing leading to a bicycle pump, and forcing air from the pump into
the tubes of the petiole and out through the leaf, which should be
held under water so as to make air-bubbles rise from the leaf surface.
70. Excretion of Carbon Dioxide from Plants. — The
oxygen absorbed by plants and distributed to all their living
cells is used in the cells in a process of slow oxidation or
chemical union of oxygen with foods and other substances
in the cells. This is the same as in animal cells. Such
oxidation is constantly going on among the particles of liv*
ing plant cells, and one of the substances formed is carbon
dioxide (CO2). When we remember that substances con-
taining carbon are abundant in cells, we can understand why
oxidation of cell-substances should form a compound of carbon
and oxygen (CO2) . For example, when sugar is highly heated
the effect is first to drive off the water and leave carbon,
which then bums and disappears in the air as a gas (CO2).
Something similar occurs in living cells when any substance
made from fat, protein or carbohydrates bums. The result
is carbon dioxide (CO2) and water (H2O). The water thus
formed cannot be distinguished from the other water which
is abundant in plant cells. The carbon dioxide is transported
(probably chiefly in solution in the moving liquids in plants)
to the surface, especially of the leaves, and then diffuses to
the surroimding air.
A plant with chlorophyll does not appear to give off car-
bon dioxide in light, because the carbohydrate-making is
using that gas much more rapidly than the cells of the same
70 INTRODUCTION TO BIOLOGY
plant are making it. If any carbon dioxide made in the cells
of the roots or of the stem is carried in the water current to
the leaves, it may be used in starch-making; and in addi-
tion, the leaves must continually absorb more of the gas
from the air. As has been stated, a closed jar containing a
measured quantity of carbon dioxide and a green plant,
placed in sunlight, will have less of the gas after a few hours ;
while a similar jar and plant kept in total darkness will
have more carbon dioxide in the air of the jar. In the first
jar, the plant used for carbohydrate-making all the carbon
dioxide produced by oxidation in its own cells and in ad-
dition some of the gas taken from the air.
71. Other Excretions of Plants. — The term excretions is
commonly applied to such substances as carbon dioxide,
which are produced by oxidation in the cells of plants and
animals, and which are eliminated because they are of no
further use in the cells, and are sometimes actually poisonous.
Carbon dioxide and water have been mentioned above as two
excretions formed by oxidation of cell-substances containing
the elements carbon and hydrogen. The water formed by
oxidation mixes with the water taken into the plant by the
roots, which is being eliminated continually by evaporation.
In addition to carbon dioxide and water, plant cells form
other excretions and also have an excess of certain substances
containing the elements absorbed with water from the soil.
In higher animals all excretory and excess substances con-
taining nitrogen, sulphur, phosphorus, calcium, etc., are in
solution in the water eliminated by the kidneys. Land plants
usually give off water only by evaporation, and certain excre-
tory and excess matters are left behind in the plant (in the
leaves) just as Ume is left in a tea-kettle. Plants living in
water may have some of these substances absorbed by the
surrounding water. Also, plants may eliminate some sub-
PLANT PHYSIOLOGY 71
stances by the roots, for the roots of some plants {e.g., oat
seedlings) will etch or corrode polished marble or limestone
by the action of acid substances which osmose from the
roots. Possibly these corroding substances enable a plant
to dissolve useful minerals. Also, some grasses give off
from their roots substances that poison and check the growth
of young orchard trees.
As plants grow older the amount of stored substance,
especially mineral matter from the soil, increases. See ref-
erence to mineral matter in old leaves (§ 58).
72. Irritability of Plants. — Irritability in either plants or
animals is the power of responding to a stimulus. For ex-
ample, if a frog be touched suddenly (mechanical stimulus),
the leg muscles contract and the animal jumps. The frog
also responds to heat stimulus . (goes into shade when the
sun's heat is too great) ; to light stimulus (sees enemies and
jumps) ; to sound stimulus (hears sounds and jumps) ; elec-
trical stimulus (jumps if touched by a slightly charged electric
wire). These are phases or kinds of irritability common in
animals which have a brain, spinal cord, and nerves.
We shall later study some microscopic animals which
respond to mechanical, heat, light, and electrical stimuli;
but they have no visible nerves or nerve organs. These
lower animals have irritability or nervous functions without
special organs to perform the functions. This is essentially
the case in plants. In recent years there have been many
magazine articles discussing " the nerves of plants." The
truth is that no one has seen any nerves or brains or similar
nervous organs in plants, although they do exhibit the vari-
ous forms of irritability and respond to the different kinds of
stimuli which affect animals, as the following examples will
show : —
Mechanical or touch stimuli (such as a jar by passing
72 INTRODUCTlOir TO BIOLOGT
animals) causes the sudden folding of the leaflets and droop-
ing of the branches of the Mimosa or sensitive plant (see
Fig. 1). The leaf of the Venus fly-trap (Fig. 2) closes quickly
and grasps insects which happen to touch it.
Response to light stimulation is seen in the familiar grow-
ing of house-plants toward the window, and also in the
opening and closing of many flowers. Flowers of the dande-
lion and others of the same family, California poppy, etc.,
open in the light and close when placed in darkness.
Heat stimulus causes many flowers to open and close;
tulip, crocus, and '^ star of Bethlehem" are familiar ex-
amples. The combined effect of heat and light stimuli upon
the opening of flowers is so marked in many species of plants
that it is possible to make a flower-clock, which is simply a
garden-bed arranged to imitate a clock-dial, in which for
each hour of dayUght there is a selected group of plants whose
flowers are commonly open at that hour. Of course, the
flowers do not open exactly on the hour, for the controlUng
temperature and sunUght vary from day to day. However,
many flowers are open in early morning; certain flowers
(example, " ten-o'clock '') open in the middle of the fore-
noon; others (like " flower-of-an-hour ") open only in the
mid-day sunshine ; " four-o'clock " and others open in late
afternoon ; and the evening primrose at sunset.
Leaves of many plants (oxalis, clovers, bean, etc.) droop
or fold in darkness and assume the so-called " sleep " posi-
tion. Some so-called ** compass plants '' avoid the intense
noonday sun by moving their leaves so that the edges are
vertical and in, the north-south direction.
Another form of external stimulus affecting plants is that
of gravitation. That the stems of most plants ordinarily
grow upward and the roots downward is a familiar fact.
Experiments made by growing young plants attached to
PLANT PHYSIOLOGY 78
rotating wheels prove that this direction of growth of plants
in a state of nature is due to gravitation.
Plants also respond to water. Roots will turn away from
dry soil and grow in the direction of greater moisture.
Still other forms of the responses of plants are the numerous
cases of the twining of stems and the movements of tendrils
and special roots in order to aid in climbing.
Tropisms. — We see that in many different ways plants
have irritabiUty and respond to external stimuli. The re-
sponses in plants are much slower than in animals, but they
are none the less definite. These reactions of plants to
stimuli are often known as tropisms (from a Greek word
meaning to turn). Turning in response to light is helio-
tropism (literally, turning to the sun), or phototropism ; to
heat is thermotropism; to gravity is geotropism (Uterally,
turning to the earth) ; to water is hydrotropism ; to chemicals
is chemotropism J' to electricity, which seems to have little
influence on plants in nature, is electrotropism. The same
terms are used in describing the reactions of animals to the
various kinds of stimuli.
Although these responses of plants to various stimuli
have been observed and studied for a long time, we do not
understand them. Neither do we know why a frog responds
when touched. We simply know that it has the power of
such response and that in some way it is connected with the
protoplasm in certain special nervous organs of the frog.
Likewise, we know many facts about plants responding in
ways very similar to animals and that this power of response
is connected with the protoplasm of plants, but apparently
in no particular organs like nervous organs. We use the
word irritability to mean that animals and plants respond to
stimuli. We do not know what it is, but there is plenty of
evidence that such a power exists, and we know much about
74 IXTBODUCTION TO BIOLOGY
how it works. Likewise, we know that electricity exists,
know much about how it acts, and can make great use of
it; and yet no one knows what electricity is. These are
merely examples out of hundreds of cases in science where
we must accept the facts and make use of the knowledge
which it is possible to discover even when we cannot find a
satisfactory explanation.
73. Summary of Work of Plant Organs : Life-Processes.
— Most plants require, asfood-^mcUerial, carbohydrates, nitro-
gen compounds, and certain other elements. Plants with
chlorophyll can make the necessary carbohydrates by com-
bining the three elements from carbon dioxide and water.
Light and chlorophyll are essential for making carbohydrates.
Plants which have no chlorophyll must absorb the carbo-
hydrates (probably as sugar) from decajdng organic matter.
Nitrogen, in the form of compoimds with other necessary
elements, is commonly obtained from the soil or water in
which the roots grow.
All plants require oxygen for use in the oxidation which is
going on constantly in all living cells. Plants with chloro-
phyll may get oxygen from that which is freed from carbon
dioxide when the carbon is used in starch-making. For this
reason such plants do not appear to take oxygen in the day-
time from the surrounding air, as they do at night, and as
animals and plants without chlorophyll do both day and
night.
All plants produce excretions by the oxidation going on in
their cells. Most prominent of these is carbon dioxide,
which all plants give off at night to the air or water in which
they live. Plants without chlorophyll give off carbon dioxide
in daylight also ; but the green plants use this gas so rapidly
when making carbohydrates that during the daytime none
appears to be given off to the surroimding air.
PLANT PHYSIOLOGY 76
All plants have assimilation or constructive metabolism of
some foods into new protoplasm. This takes place only in
living cells. Much of the food containing only carbon,
hydrogen, and oxygen (i.e., carbohydrates and oils) is be-
lieved to undergo oxidation in cells or is stored for future
use, but does not become protoplasm. Probably only a
small part of the contents of an ordinary plant cell is living
matter, and much of the cell-substance which we see with
the microscope consists of food-materials, water, and other
lifeless substances.
Digestion of foods is necessary whenever insoluble foods
(starch, oil, proteins) require transfer from cell to cell or to
distant organs of the plant.
Moving liquids in the higher plants serve to transport the
foods, oxygen, and excretions ; but these liquids do not make
a complete circuit as do the animal circulating liquids (blood
and lymph), whose function is also transportation of foods,
oxygen, and excretions.
Some form of irritability, or power of responding to stimuli,
is present in all plants. But there are no special nervous
organs, such as are connected with irritability in higher
animals.
All species of plants have the power of reproduction. This
will be considered in Chapter XII.
CHAPTER V
STUDY OF INSECTS: INTRODUCTORY LESSONS
IN ANIMAL BIOLOGY*
74. Study of Insects. — The word Insecta is the scientific
name for a large group of animals popularly called insects,
and of which familiar examples are grasshoppers, cockroaches,
flies, beetles, bees, and butterflies. Spiders are also popularly
regarded as insects; but we shall see in later studies that
they are different from all the familiar insects.
It is scarcely possible to find for beginning the study of
zoology any animals more interesting than insects. Many
insects have useful or injurious relations to man ; e.g., some
destroy plants, some transmit the germs of disease to man
and domesticated animals, and some produce such useful
substances as honey and silk. All such relations of insects
to man are said to be economic, and they will be considered
in §§ 260-262, 319, 320. Aside from the domesticated birds
and mammals, the insects are of more economic importance
* To Teachers : This study of insects may precede Chapters III and IV
(see §§ 32-64 and 65-110 in the "Teachers' Manual of Biology "). Septem-
ber and October are good months for study of insects out-of-doors; but
many of those most useful for this study may be kept in insect-cages (see
the •* Teachers' Manual of Biology," § 323) ; and thus make it possible to
use the first five or six weeks of the school year for the study of plants ac-
cording to the two preceding chapters. Natural history of insects is interest-
ing and especially desirable for students who have not 'had the advantage of
nature-study in the elementary school ; but in high schools most of the-out-
door study of living insects must be supplementary to regular work planned
for laboratory and classroom.
76
INTRODUCTORY LE880N8 IN ANIMAL BIOLOGY 77
than are all other groups of animals taken together. But
quite apart from practical matters, the insects have long
been favorite objects for study because many are beautiful,
many have remarkable adaptations to special conditions of
life, and many have wonderful instincts and nervous activi-
ties that are surpassed only by certain birds and mammals.
More than half of the known species of animals are insects.
At least 250,000 species are now known, and many newly
discovered species are named and described each year.
Some experts in entomology (that division of zoology which
deals with insects) believe that there may be living now a
million species of insects.
In order to observe living insects satisfactorily, one must
first know the general plan of insect structure and the uses
or functions of the main parts or organs of their bodies. It
is a fortunate fact for students that insects are all built on
the same plan of structure, and that a study of a few selected
specimens will enable one to understand almost any other
insect which may be seen. Some of the many kinds or
species of grasshoppers, flies, and butterflies are commonly
used for introductory studies ; but many other conmion in-
sects might serve well for first lessons.
76. Study of a Grasshopper. — Materials. — For study of struc-
ture, either specimens recently killed (by chloroform, benzine, or
cyanide) or some preserved in alcohol or formalin solution. Living
grasshoppers in glass jars (e.g,, jelly glasses with perforated tin
covers) with some green grass or other plants for food. Crickets,
cockroaches, or katydids may be used as substitutes for grasshoppers,
or for comparison.
Laboratory Study. — Place a grasshopper, or a similar insect, so
that you can easily see its parts, carefully read the following labora-
tory directions and make sure that you see and understand each
point before passing on to the next.
(L) The grasshopper consists of body and of legs, wings, jaws,
find other projecting parts which collectively are called appendages.
78 INTRODUCTION TO BIOLOGY
Notice three regions of the body, — head; thorax, with three pairs
of legs ; and a segmented or jointed abdomen. Notice that the body
and appendages have a hard covering, and external skeleton. There
is no internal skeleton as in the human body.
Just as in geography we use the terms north, south, east, and
west to indicate directions, so in zoology we must have terms for
directions or positions on bodies of animals. The head-end of a
grasshopper is called anterior, the oppo-
site end of the trunk is posterior, the
lower surface of the body is ventral, and
the back or upper surface is dorsal.
Any animal having differentiated
(meaning ** made different ") dorsal and
ventral surfaces, and anterior and pos-
terior ends, might be divided into
similar halves (right and left) by cut-
ting in the median plane from anterior
to posterior and from dorsal to ventral.
^'I'JL ^JTJiZr^t^. Can you think of any other plane in
hopper, a, i, antenor wing; , . / , ,
^, posterior. (After Joutel.) which a grasshopper can be equally
divided ? Such an animal is bilaterally
symmetrical. Do you know any animal which is not so? Animals,
like jellyfishes, which have the wheel-like plan of structure are said
to have radial symmetry. There are comparatively few such ani-
mals, but many plants are so arranged.
The word organ will be used frequently hereafter. An organ is
a part of the body of a plant or animal fitted for doing a particular
work. For example, the heart is an organ for pumping blood, lungs
are organs for breathing, muscles are organs for moving. A group
of organs doing similar work form a system. Thus, all the muscles
constitute the muscular system, which is a group of organs adapted
to the work of producing movement.
Abdomen. — The tip of the abdomen of the female bears two pairs
of pointed structures which are of use in depositing the eggs, and
together they are known as the ovipositor. The end of the abdomen
of the male is turned upward. Count the rings or segments in the
abdomen. On the first ring next to the thorax there is on either
side a shining oval patch, the organ of hearing. Along each side of
the abdomen is a groove, and just above it is a row of pores (spiracles
or breathing pores). Watch the breathing movements of the ab-
INTRODUCTORY LESSONS IN ANIMAL BIOLOGY 79
domen of a living grasshopper. The spiracles are connected inside
with a system of branched air-tubes {trachecE), which ramify through
the body and distribute air directly to the tissues. Compare this
method of breathing, which is characteristic of insects, with that of
the human 'body.
Thorax. — Each pair of jointed appendages (legs) represents a
segment of the thorax. How many pairs of legs are there ?
Look for the breathing pores on the second and third segments.
Examine the two pairs of wings
and compare them as to form,
size, texture, color, position, and
use. The veins in the wings are
hollow tubes which carry blood
and air.
The characteristic shrill sound
made by katydids is caused by
rubbing the upper wing on the
lower wing.
Compare legs from each seg-
ment of the body which bears
them. Are they similar? Ob-
serve the hooks and pads on the
feet — these are used in clinging
when the animal is at rest. Examine the legs that are adapted for
jumping.
Head. — The head is attached to the thorax by a soft neck.
Examine the large eyes with a hand-lens, and with a microscope ex-
amine a slice from one of the eyes. These characteristic insect eyes
are compound. Also, many insects have several bead-like simple
eyes (ocelli) situated on the head between the compound eyes (use
hand-lens). Examine the long slender feelers {antenncs) with a
hand-lens. Examine the mouth-parts; upper lip (labrum) ; lower
lip (second maxiUsB, or labium) ; first maxillsB beneath the upper
Up; jaws (mandibles) between the lips; and a tongue-like organ
within the mouth.
Make enlarged drawings of a grasshopper : (1) of side view with
wings in closed or resting position, (2) of front view of head, (3) of
a leg, (4) of side view of abdomen.
Examine young grasshoppers of various sizes. How do they
differ from the adult specimens ?
Fig. 23. a, egg-laying
hoppers; 6, eggs; c, d,
eggs. {After Riley.)
of
of
80 INTRODUCTION TO BIOLOGY
Observe method of locomotion, taking of food, kinds of food,
and breathing movements of grasshoppers kept in glass jars. When
opportunity offers , carefully observe them in the fields. Record your
observations.
Grasshoppers are called locusts in the Old World, but rarely so
in America. The so-called " seventeen-year locusts " are really
' cicadas, belonging to an entirely separate order of insects (the Hemip-
terans).
76. Study of Fly. — The house-fly is most easily obtained, but
some of the larger species are better for laboratory study. Pieces
of fresh meat left outdoors for a few hours will probably attract
some of the " blue bottle " or " blow flies " ; and they will lay eggs.
These will hatch within 24 hours, and the larvae (** maggots ") begin
to eat the meat. Each day they may be brushed from the old to
new pieces of meat, and thus avoid disagreeable odors. It is said
that they may be fed with moist bran, oatmeal, or even soaked
crackers. The larvae are full grown within a few days and then
pupate. The pup» resemble small, brown seeds. Finally the
pupal case breaks at one end and the perfect fly emerges. The
house-fly pupa changes in about one week.
Flies do not grow after they reach the fly stage. Their formation
of tissue takes, place while they are voracious larvae. The flies of
many sizes often seen in houses are not young and adults, as com-
monly supposed ; but all are adults of various species. The food
of the larvae is not the same for all species. The blow-flies com-
monly lay their eggs on meat ; but the house-flies lay their eggs in
manure and various kinds of filth. If disease-producing bacteria
happen to be present in the filth on which the house-flies walk,
these bacteria may later be left on foods. The piercing flies may
inject certain bacteria into the blood of humans and animals.
These relations of flies to disease will be considered in § 261.
(L) Examine a fly as you did the grasshopper, noting head,
thorax, abdomen, legs, and wings. Does the fly have the same num-
ber of wings and legs as has the grasshopper? With a hand-lens,
examine (1) the eyes; and (2) the mouth-parts (in some species
adapted for rasping and lapping, in others for piercing and sucking).
Examine the feet with hand-lens.
Examine a larva with hand-lens. The head is small, the body is
segmented, and there are no feet. Examine a pupa. Only the
brown protective case can be seen.
INTRODUCTORY LESSONS IN ANIMAL BIOLOGY 81
.-^^i/^^^^n ^Buti*f^if
T/wrar vf BuHetit^
*Spir9ctm
11, Study of Butterfly, or Moth. — (L) Examine a butterfly
with regard to the points of structure already seen in a grasshopper.
The chief differences to be observed are: (1) Large wings covered
with scales (use hand-lens and microscope). On some butterflies
and on moths there are hair-like scales. (2) Legs of butterflies are
adapted for clinging, not jumping. (3) AntennsB are club-shaped on
a butterfly, feather-like on a moth. (4) Mouth-parts are adapted
for sucking nectar of flowers. There are no jaws for biting solid
food, 'but a long
coiled tube, consist-
ing of two halves
closely applied to-
gether, is the suck-
ing organ. How-
ever, the butterfly
is, on the whole,
built on the grass-
hopper plan of
structure.
Larva, — Most re-
markable about but-
terflies and moths
is their pecidiar de-
velopment from egg
to larva (caterpillar), then to pupa (which may be in a silky cocoon) ^
and then to perfect insect {imago).
The larva called "tomato-worm" (which develops into a hawk-
moth) is excellent for study. Observe : (1) Head, and its parts. (2)
Thorax, with three segments, each having a pair of legs. (3) Next are
some segments with no legs, and tben come several segments with pe-
cidiar legs adapted to clinging to twigs (prop legs) . (4) A curved spine
at posterior end. (5) Spiracles or breathing pores along the sides
of the body. (6) Color markings in living specimens, if available.
Make an outline drawing of the larva as seen from its right side.
Pupa, — The pupa formed from the tomato-worm is buried in
the soil and difficult to find. Pupa from cocoons of Cecropia moth,
or other large moths, may be used. The cases or covers of various
organs, as shown in Fig. 25, may be seen. Identify the covers of
the tongue, antennaa, legs, eyes, and wings. Note spiracles on sides
of the body. Examine the movable segments of the abdomen.
Jfea<i
Fig. 24. Butterfly larva. {From Dickerson's
** Moths and Butterflies*' Ginn & Co.)
82
INTRODUCTION TO BIOLOGY
The fragments to be found in a cocoon at posterior end of a pupa
are from the skin which was molted after the cocoon was spun'.
The larva molts several times before it is full
grown and ready to change to pupal stage.
Cecropia moths will emerge in February or
March if cocoons are kept in a warm room,
and much later if left outdoors. The habits
of the living moths should be observed. They
will live but a short time, for their alimentarj'
canal is too imperfect to use food. This is
true only of some of the species of moths and
butterflies.
Larva, pupa, and adidt stages of common
butterflies and moths can be identified by the
figures in Dickerson's "Moths and Butter-
flies," which also gives very readable accounts
of the life-histories.
Fig. 25. Pupa of
moth, a, antenna
case; w, wing case;
tongue and leg
cases between an-
tennse ; a6, abdo-
m e n . {From
Kingsley.)
78. Physiology of Insects. — These animals have the
following systems of organs : supporting (the external skele-
ton) ; muscular (muscles of the body and appendages) ;
digestive (mouth, esophagus, stomach, intestine, digestive
glands) ; circulatory (heart and blood-vessels) ; respiratory
(air-tubes) ; excretory (some excretions are in the indigestible
materials discharged occasionally from the intestine) ; ner-
vous (simple brain in head, a double nerve-cord in ventral
part of the body, and nerves extending to various organs of
the body) ; and the reproductive organs (ovaries and ovi-
ducts in female, spermaries and sperm-ducts in male).
The supporting of the body of the backboned animals is
accomplished by an internal skeleton, while an external case
or skeleton serves the same purpose in an insect.
The digestive organs prepare food for absorption.
Circulating blood carries digested food, oxygen, and
excretions; and a heart provides the motive force of
circulation.
INTRODUCTORY LE8S0N8 IN ANIMAL BIOLOGY 83
The air-tubes of an insect perform the same work as the
lungs of the human body.
The nervous system controls the activities of all organs.
The reproductive organs — the ovaries form ova or egg-
cells, the spermaries produce sperm-cells, while the oviducts
and sperm-ducts are simply tubes for conducting egg-cells
and sperm-cells to the exterior, where each egg-cell may be
entered and fertilized by a sperm-cell and then develop into
a new individual.
79. Molting of an Insect. — It is evident that an animal
inclosed in a hard external skeleton cannot grow rapidly
while surrounded by such a coat-of-mail. This diflEiculty is
overcome by periodical shedding of the skeleton, followed by
rapid increase in size for a short time while the skin remains
soft and extensible. The shedding or molting occurs nine
or ten times before the grasshopper is full
grown. The process of molting is briefly
as follows: The skeleton splits along the
middle of the back, and through the split
the head first emerges and then the pos-
terior end of the body. The struggles to
get free from the old skeleton are often so
violent that the animal becomes exhausted
and dies. The wings are larger after each
molting.
80. Life-Histories of Insects. — The eggs
of the simplest and lowest insects {e.g., the
silver-moth. Fig. 26) form young which at
hatching are Hke the adults except in size.
The young of grasshoppers and of numer-
ous other species of insects are at first with-
out wings, and they go through a number
of moltings of the external skeleton before
Fig. 26. A "silver-
moth," Lepisma.
A modern repre-
sentative of the
ancient primitive
insects which had
three pairs of legs
but no wings.
84
INTRODUCTION TO BIOLOGY
Fig. 27. Firefly
larva {1) and
adult (;?). ReaUy
a fire-beetle, for
it is not closely
related to flies.
{From Packard.)
they become adults. This condition is called gradual metamor-
phosis, or incomplete metamorphosis. The most complicated
development is represented by the butterflies, moths, beetles,
flies, bees, and wasps. The eggs of these
hatch into worm-like larvce (popularly called
caterpillars, grubs, maggots). These larvae
are voracious feeders, and usually grow
rapidly to full size. Then while transform-
ing from the larva to the adult or imago
stage, they undergo a period of quiet in the
pupa stage. The pupa stage of the butter-
fly is often called a chrysalis. The larvae
of some insects (e.g., silkworm and many
moths) spin a protective cocoon around
themselves as they pass into the pupa stage. After a period
of quiet, during which vast internal changes (complete meta-
morphosis) are taking place, the pupal case bursts and the
perfect insect emerges
in full-grown state.
Many insects live
longer in the larval
than in the adult
stage. Some of the
May-flies (also called
day-flies and ephem-
erids) live a year in
the larval stage and
only a day or two
as perfect insects. Vast numbers emerge in the morning,
soon lay their eggs, are unable to take food, and perish before
the next day. The seventeen-year cicadas (erroneously
called " locusts," for this name belongs to grasshoppers)
spend the greater part of the seventeen years as larvae
Fig. 28. Stages in the gradual metamorphosis
of squash-bug. {From FoUom.)
INTBODUCTOBT LESSONS IN ANIMAL BIOLOGY 85
attached to roots of trees. In the seventeenth summer they
come to the surface, soon pass through the pupal stage, and
then emerge as perfect insects. A few of them may remain
for a number of weeks, but vast numbers lay their eggs in
the twigs of trees and die within a few days after becoming
adult insects. The larvae, which hatch from the eggs de-
posited in the twigs, fall to the ground, burrow, and remain
in the soil for nearly seventeen years. A thirteen-year
variety occurs in the Southern States.
The insects which have complete metamorphosis are re-
garded as the highest. The complicated life-history has
many advantages. The insect in the larval stage is often
able to conceal itself and to reach food which perfect insects
could not get ; for example, the larvae of numerous species
of beetles, of the cicadas, and of flies. In many cases it is
possible for the adult insect to supply food for the larvae;
for example, bees supply food to the larvae in the honey-
comb, ants feed and care for their larvae, some wasps paralyze
other insects by stinging and then place them as food for
wasp larvae, and the eggs of many parasitic species are laid
in other insects. Still another advantage to some insects is
the fact that their larvae easily withstand the winter storms
(examples are many moths with cocoons). Also, there is an
advantage in that the larvae live under conditions different
from the adult insect ; for example, numerous larvae feed on
foliage which the adults never frequent except for laying
eggs.
81. Classification of Insects. — The insects constitute a
subdivision of the animal kingdom known as Insecta and this
is divided into about twenty groups called orders.
In classifying insects, as all other animals, it is necessary
to consider all points of external and internal structure in
order to determine which kinds or species are most aUke ;
86 INTRODUCTION TO BIOLOGY
but it is a fortunate fact that it is often possible to identify
many species by means of some one prominent structure.
In many insects, the wings happen to be the convenient
parts for general classification ; but similar wings must not
be taken as indicating close relationship if other organs are
not homologous (i.e., of corresponding structure).
The most common insects in the United States are con-
veniently grouped in the following orders : * —
Aptera. — Name means without wings. Example i the
" silver-moth " or " silver-fish " (Fig. 136) and the '' spring-
tails." These are the oldest insects now Hving. They are
primitively wingless ; that is, there is no evidence that their
ancestors had wings. Fleas are secondarily wingless, because
of degeneracy caused by parasitism. Entomologists con-
clude that the ancestors of fleas had wings because embryo
fleas show the beginning of wings. Hence, such degenerate
insects are not apterans, but belong in a higher order.
Orthoptera. — Name means straight wings, referring to
main veins of the wings, or to the way they fold together.
Examples: grasshoppers, crickets, katydids, stick-insects,
cockroaches, mantis, earwig. Two pairs of wings, similar
to those of grasshopper. Mouth-parts for biting. At least
10,000 species are known.
Netted-winged Insects. — Old books recognize an order
Neuroptera for all insects which have two pairs of netted-
veined wings like the dragon-fly and May-fly ; but study of
their other organs and especially of their embryonic develop-
ment has shown that dragon-flies, May-flies, termites (often
called white ants), and others with netted wings are not
similar except in their wings. The netted-veined wings are
found on insects now grouped in five or six orders. How-
* These are easily learned by examining and comparing specimens.
INTRODUCTORY LESSONS IN ANIMAL BIOLOGY 87
ever, the beginner cannot do better than to use the popular
name " netted-veined," and for further information consult
the larger books on insects for descriptions of Neuroptera
and other orders of insects which exhibit this kind of
wings.
Hemiptera, — Name means half wings, referring to the
fact that in some of these insects about half the wing next to
the body does not show distinct veins. Examples : all true
bugs, cicadas, squash-bug, box-elderbug, " stink-bug,'' chinch
bug, parasitic Uce, bed-bug, water-bug, cochineal bug, plant-
lice, scale-bugs. Four wings, overlapping when folded.
Mouth-parts for piercing. Incomplete metamorphosis.
More than 20,000 species.
Lepidoptera, — Name means scale-wings, referring to over-
lapping scales. Examples: butterflies and moths. Two
pairs of wings, covered with flat or hair-like scales. Mouth-
parts for sucking. Complete metamorphosis. Butterflies
usually fly in daytime and have slender antennae with knob
or club at end. Moths are nocturnal and have feather-Hke
antennae. Over 50,000 species of Lepidoptera are known.
Coleoptera. — Name means sheath wing, referring to the
hard anterior wings. Examples : all beetles, — weevils, fire-
flies, June beetles, blister beetles (" Spanish flies '0- Some
so-called bugs, e.g., lady-bird " bug " and June " bug," are
beetles and not bugs (i.e., not Hemiptera). Front wings of
beetles are hard and homy, and often called the elytra.
Hind wings are membranous. Complete metamorphosis.
It is estimated that over 100,000 species of beetles have been
named.
Diptera. — Name means two wings ; hind wings absent.
Examples : flies, bot-flies, warble-flies, mosquitoes. Mouth-
parts for sucking (as in house-fly) or piercing (as in horse-
flies). Complete metamorphosis. Larvae commonly called
88 INTRODUCTION TO BIOLOGY
*' maggots." Small knobs represent the hind wings. More
than 40,000 species are named.
Fleas, — Parasitic insects, wingless or with rudimentary
wings. Formerly considered wingless flies, but now in a
special order.
Hymenoptera, — Name means membrane wing, e.g., a bee's
delicate wing. Examples: ants, bees, wasps, ichneumons.
Mouth-parts for both biting and
sucking. Complete metamorphosis.
Four wings, hind pair smaller, few
irregular veins. Probably about
30,000 species known.
Practice in Classification of Insects, —
(L) Students should examine a set of
insects (a mixed lot preserved in wood
Fig. 29. An ichneumon, ^l^ohol or formalin is best) ; and by
a, antennse; o, ovipositor; eomparmg with above descriptions, and
3, third legs. (After the well-known examples mentioned,
Riley.) assign the specimens to the proper
orders. In an hour of time one can
learn to identify as to orders the most common insects. The names
of genera and species of the very common insects can be found in
the large books, such as Comstock's " Manual of Insects," but
rarer specimens can only be identified by specialists.
Students who are specially interested should read some of these
books : Dickerson's ** Moths and Butterflies," Comstock's " Insect
Life " and ** How to Know the Butterflies," Sanderson and Jack-
son's " Elementary Entomology," Smith's ** Insect Friends and
Enemies." Kellogg's " American Insects," Holland's " Butterfly
Book " and " Moth Book," and Folsom's " Entomology " are more
advanced, but even beginners can use their illustrations.
82. Economic Relations of Insects. — Many species of
insects are of interest because they have economic or prac-
tical relations to man. Many species destroy cultivated
plants (see § 320). A very few species, such as the honey-
INTRODUCTORY LESSONS IN ANIMAL BIOLOGY 89
Fig. 30. Photograph of stages in life-history of ants reared in
captivity by Adele M. Fielde. 1, cocoons; 2, young ant
emerging from cocoon; 3, 4. males; 5, workers; 6, several
larvae ; 7, naked pupa ; 8, winged " queen." {From
Naiure-Study Review, 1905.)
90
INTRODUCTION TO BIOLOGY
-^
FiQ. 31. Water-boatman,
legs adapted as oars.
Third
bee and silk-worm, are useful for their products (§ 319).
Many species transmit the germs of destructive diseases
to domesticated animals. Flies and mosquitoes introduce
into the human body the germs
of typhoid, malaria, and other
diseases (see §§ 260-262).
83. Adaptations of Insects.
— No other group of animals
is so favorable as the insects
for study of adaptations of
structures td special uses.
Insect legs are adapted to running (cockroach), leaping
(grasshopper), walking (fly), grasping (mantis), burrowing
(mole-cricket), clinging (moths and butterflies), carrjdng
pollen (bees), and to still other special uses. However, in
all these cases the general plan of a leg is similar.
The mouth-parts are adapted to various kinds of foods;
biting and chewing (grasshopper, beetles), piercing (mos-
quito, hemipterans, and some flies), licking (house-fly), suck-
ing (butterflies),
and cutting (car-
penter-bee).
The wings are
commonly well
adapted to flying,
but in some species
(e.gf., fleas) wings
would be worse than useless organs and have become
degenerate. The front wings of beetles are thickened to
form shield-like covers for the posterior wings. In protec-
tive adaptation of some insects the wings are remarkably
leaf-like in appearance.
The form of insect bodies is strikingly modified in many
Fig. 32.
Mantis. First legs are adapted
catching other insects.
INTRODUCTORY LESSONS IN ANIMAL BIOLOGY 91
species. Stick-insects (Phasmidae) are orthopterans in
which the body and legs are stick-like and easily mistaken
for sticks, twigs, straws, etc., near which
these insects live.
Some of the most remarkable adapta-
tions are connected with the larval stages.
Larvae of May-flies and others live in water
and breathe by means of feathery gills on
the abdomen (Fig. 34). The larvae of
mosquitoes have special breathing tubes
which can be extendied above the surface
of the water. Larvae of lepidoptera which
crawl on plants have false legs for support-
ing the posterior end of the body; but
those of flies, beetles, ants, bees, etc., de-
velop in situations where such extra legs
are not required and they have only the
three pairs of thoracic
legs.
In various ways protec-
tive structures for larvae
are formed, and organs
are specially adapted for such work. Ex-
amples are : spinning silky cocoons (by
moth larvae), making honeycomb (by adult
bees), making paper (by adult wasps),
making cases or tubes from bits of plants
or gravel (by caddis larvae), and roUing
leaves to form tubes (by many larvae).
Aquatic insects are especially noteworthy
for their adaptations. In some species
oar-like legs are fitted for locomotion by swinuning. Mos-
quito larvae propel themselves by " wriggling '' of the abdo-
Fig. 33. A stick-in-
sect, a, anteniiEe;
w, mouth ; 1, 18,
5, three pairs of
legs.
Fig. 34. Larva of
May-fly, with
feathery gills along
side of the abdo-
men adapting to
aquatic larval life.
92 INTRODUCTION TO BIOLOGY
men. Some aquatic larvae (e.g., mosquito pupae) are so
filled with air that they float at the surface and can descend
only by swimiming. A velvety covering of hairs on the
body and legs enables some insects to skate on the surface
film of water. The silvery white of the " backswimmers " is
due to a layer of air held by delicate hairs. Other aquatic
beetles and bugs have air-spaces beneath the front wings.
Adaptations of insects with respect to color are so impor-
tant that a separate section of this chapter is devoted to
them.
84. Colors of Insects. — Many insects are colored in ways
which are apparently useful to them. Such useful colors are
commonly interpreted as (1) protective, (2) aggressive,
(3) warning, (4) mimicry.
(1) Protective coloration is the most common form of useful
colors among insects. A green insect on a green leaf is
more or less concealed because it so closely resembles its
surroimdings. Many insects resemble the bark of trees on
which they rest, and some both in form and color resemble
sticks and leaves. In still other ways numerous insects re-
semble the objects near which they habitually live. Such
similarity to the environment is believed to be protective
against enemies. Note that it is not absolutely protective,
for such insects are often captured by birds ; but probably
more such insects escape than would if they were not pro-
tectively colored.
(2) Aggressive coloration is the term applied to insects
which resemble their environment so that they can he con-
cealed from their approaching prey. This is found in certain
predaceous insects like the mantis, which at the same time
are protectively concealed from such enemies as birds.
(3) Warning Coloration. — Many insects are conspicuously
colored, and appear to make no attempt at hiding them-
INTRODUCTORY LESSONS IN ANIMAL BIOLOGY 98
selves. The common monarch butterfly is an example.
The explanation is that such insects often have a disagree-
able odor, flavor, or sting which repels their enemies ; and
hence the conspicuous color is a danger signal.
It has been learned by experiments that insect-eating
monkeys soon learn that such a conspicuous coloration is
associated with disagreeable things and thereafter will not
attempt to catch such insects. This suggests that a species
with warning colors would have an advantage in that the
comparatively few individuals caught might teach the enemy
that such brightly colored insects are not good to eat.
(4) Mimicry. — This means the resemblance of one species
of insect to another which has warning colors. The Ameri-
can viceroy butterfly is a mimic of the monarch. Young
insect-eating monkeys will eat viceroy butterflies, but will
not touch them if they have first tasted some unpalatable
monarchs. This leads to the view that the viceroy species
gains decidedly by resembling and thus living on the bad
reputation of the monarch, which has warning colors. In
tropical countries there are numerous similar cases where a
conspicuous insect' which has no special defense against its
enemies is similar in appearance to another species whose
conspicuous colors are warning signals, advertising boldly to
enemies that there is danger.
The word " mimicry " suggests conscious imitation ; but,
of course, one insect resembles another because it happened
to be developed that way. We do not know how the first
viceroys came to resemble monarch butterflies, but probably
a butterfly appeared which was unlike its near relatives and
more like monarchs. This resemblance to the monarchs
gave the first viceroy an advantage. Other viceroys de-
veloped because " like tends to produce like," which prin-
ciple of embryology applies especially to the propagation of
94 INTRODUCTION TO BIOLOGY
individual organisms bom with some peculiarity. Obviously,
any slight advantage gained by the resemblance to the mon-
archs, which are not so liable to attack as are more edible
insects, would have tended towards the continued multi-
plication of viceroys. It is thus easy to suggest how viceroys
might have been preserved and allowed to multiply ; but it
has not yet been discovered why the first viceroy happened
to be so unlike its relatives.
The above explanation of insect colors as protective,
aggressive, warning, and mimicking is believed to apply to
many insects. It should be understood that these devices for
the use of colors are not absolutely perfect in their working.
For example, green grasshoppers concealed in green leaves
are often discovered by birds ; but there is reason to believe
that the green color makes it harder for birds to find the
insects, and hence the color gives some protective advantage.
Likewise, warning colors are not always efficient danger
signals; and mimicry does not always deceive enemies.
However, there is probably a decided advantage, on the
whole, to species of insects which possess any of these forms
of coloration.
85. Colors of Other Animals. — Animals other than in-
sects may have useful coloration. Thus fishes, frogs, lizards,
and many other animals may have concealing coloration
which may work either protectively or aggressively. The
white of birds, hares, and other small animals on arctic snow-
fields is believed to be protective ; while the white of polar
bears may be aggressive, enabling them to lie in wait for the
approach of prey. The stripes of tigers are supposed to be
aggressive because they harmonize so well with the lights and
shadows among the reeds of jungles. Likewise, the tawny
color of the lion resembles the sands of the desert.
Some of the most remarkable cases are those of frogs
INTBODUCTORY LESSONS IN ANIMAL BIOLOGY 95
and lizards, which can quickly change colors when placed on
new objects, e,g., from green to brown when moved from a
leaf to the bark of a tree. This is accomplished by a peculiar
arrangement of pigment-cells in the skin. Some cells contain
green pigment and some have darker colors. When the
green cells are expanded ^o as to expose their maximum sur-
face, and the dark cells* are contracted, the animal's skin
appears green. Expanded dark cells and contracted green
cells make the skin brown in color. Partial expansion and
contraction of both kinds of pigment-cells give intermediate
shades. The cells are controlled through the nervous system
and the eyes.
In the case of those birds and mammals which appear to
have concealing colors, it should be remembered that many
of the enemies have a keen sense of smell, and so colors can-
not always be useful. For illustration, quails and other
birds which live on the ground are more or less protectively
colored. This may help them some in escaping such enemies
as hawks, but obviously would be of no protection against
weasels, skunks, and other enemies which hunt by night,
guided by odors and not by colors.
Finally, it should be noted that some colortS of animals are
probably not useful. For examples, we may mention the
beautiful colors concealed in shells of some kinds of moUusks,
the " eye-spots *' in wings of some moths and butterflies,
and the gorgeous color-patterns of many birds (e.g,j pea-
cock). For all of these and very many more we have as yet
no satisfactory explanations.
86. Instincts of Insects. — The instincts of insects have
long aroused the wonder of students of animal life, for under
the guidance of inherited instincts many insects exhibit re-
markable behavior. It is instinct which leads honey-bees
to build honeycomb, care for the young in true nurse fashion,
96 INTRODUCTION TO BIOLOGY
give a different kind of food to the larvae destined to be-
come " queens," follow the " queen " when she leaves the
hive at swarming time, sting intruders, and in various
other ways behave almost like intelligent beings. It is
instinct which leads parasitic species to lay their eggs in
the right kind of larvae, and other species to place the eggs
on plants which will furnish the right kind of food for the
yoimg larvae. It is instinct which leads certain wasps to
capture and paralyze other insects, and then place them
where wasp larvae hatching from eggs can eat them later.
In short, thousands of cases of striking behavior of insects
are apparently due to instinct. Some of the actions of in-
sects suggest that they are intelligent and capable of reason-
ing ; but the most critical studies have led entomologists to
the conclusion that insects do not reason, but that they con-
stantly act instinctively and automatically. How they
originally acquired their instincts is entirely unknown, but
that the instincts are transmitted from generation to genera-
tion is certain. A bee does not have to learn to build honey-
comb, for to do this is as natural as eating and moving. It
is an instinctive action.
Some insects seem to have ways of communicating with
each other. An ant or bee may, while wandering around,
find something good to eat, and soon many others will follow
the first one from the nest. This is probably due to the
odor of food conveyed to the nest by the first insect. The
ability of ants to recognize those from another nest is merely
distinguishing between odors of different nests.
87. Classification, or Position of Insects among Animals.
— The animal kingdom is composed of various types of
animals. The highest and most complex in structure are the
backboned animals or vertebrates j so named because the back-
bone is a series or a row of bones, each of which is called
INTBOnUCTOBY LESSONS IN ANIMAL BIOLOGY 97
a vertebra. Common types of vertebrates are fishes, frogs,
reptiles, birds, and mammals (dog, horse, sheep, seal, monkey,
etc.). Simpler in structure and therefore lower than the
vertebrates are the backboneless animals (invertebrates) j ex-
amples of which are : moUusks (clam, snail, oyster), insects,
crustaceans (crayfish, lobster, crabs, shrimps), worms, jelly-
fishes, sponge-animals, and the simple animals revealed by
the microscope. The insects are among the highest of the
backboneless animals. They are similar to spiders and to
crustaceans, especially in having an external skeleton and
legs with many joints ; and because of similarity the insects,
spiders, and crustaceans are placed together in a great group
known as the arthropods (scientific name, Arthropoda).
Chief Groups of Animals and Plants. — In the table below
the lowest group of the animals is named first and the highest
last, but there is doubt as to the relative position of some of
the intermediate groups ; for example, whether annelids are
higher than echinoderms, or arthropods higher than mollusks.
Each of these primary groups of animals is usually called a
phylum (plural phyla), e.g.. Phylum Protozoa. The advanced
textbooks of zoology include about fifteen phyla of animals.
The common animals belong in the ten phyla named below.
The main subdivisions of these phyla are given at the ends
of the chapters in Part III of the " Applied Biology."
Protozoa (simplest, OHe-celled animals).
Porifera (sponge-animals).
Coelenterata (hydroids, jellyfishes, coral-animals).
Platyhelminthes (flat worms, tapeworms).
Nemathelminthes (round worms).
Annelida (segmented worms).
Echinoderma (starfish, sea-urchin, crinoid, sea-cucumber).
Arthropoda (lobster, crab, spider, centipede, insect).
Mollusca (clam, oyster, snail, cuttle-fish).
Vertebrata (backboned animals).
CHAPTER VI
SOME ANIMAL STUDIES PRELIMINARY TO HUMAN
BIOLOGY: VERTEBRATES
The purpose of this chapter, and of the two that follow
it, is to teach the chief facts regarding the structure and
functions of some animals which help us to an understanding
of the biological study of the human body in Chapter VIII.
Study of any animal or plant is helpful in preparation for
study of the human body; but the vertebrate animals are
especially helpful because in their general structure and
functions they are very much like the human body. For
this reason, the present chapter is devoted to certain verte-
brates. The next chapter deals with certain lower animals
which illustrate more simply than do the vertebrates prin-
ciples of biology that apply to the human body. Finally,
in Chapter VIII the human body will be considered in the
light of the studies of animals in Chapters VI and VII, and
of plant physiology in Chapter IV.
Among the backboned animals none has been so popular
for scientific study as the conmion frog. Many books, some
of them very large, have been written about the biology of
this animal, and the scientific knowledge concerning it is
greater than that on any other animal. Strange as it may
seem, we know far less concerning the human body from
direct study; but fortunately the frog and the human are
so much alike in numerous ways that biologists have applied
to the human species many facts which were learned first
98
VERTEBRATES 99
by study of the frog. Since the study of the frog by scien-
tific men has played so important a part in building up the
science of animal biology, teachers now regard this animal
as valuable for study by those who wish to rediscover for
themselves some of the most important facts concerning
animal structure and life.*
Readers of this book who have not studied, in elementary
school nature-study, the life of the conmion toads and frogs
should read at least one of the following : '^ Usefulness of the
American Toad," in Farmers' Bulletin, No. 196 (free from
U. S. Dept. of Agriculture) ; '^ Life History of the Toad," in
Cornell Nature-Study Leaflets; or Chapter 16 in Hodge's
" Nature-Study and Life."
I. THE STRUCTURE (ANATOMY) OF THE FROG
OR TOADt
88. External Structure of the Frog. — (L) Place a living frog in
a clear glass tumbler, and cover with paper or mosquito-netting.
The ordinary ** jelly-glasses " with tin covers are convenient, if
small holes are punched in the cover. By looking through the glass,
it will be possible to learn many things about the frog's external
structure and habits.
Notice that the frog's body consists of head, trunk, and limbs.
How many limbs ? Is there a neck, or a tail ?
Imagine your own body supported on all four limbs (i.e., walking
on hands and feet) and locate anterior, posterior, dorsal, and ventral.
♦ To Teachers : Before beginning any dissection of animals, it will be
helpful to give the students the point of view of § 34, "Justifiable Use of
Animals for Science Study," in the *' Apphed Biology."
t To Teachers : Concerning the importance of demonstrating the struc-
ture of either frog or toad, in case individual work by students is impossible,
see § 35 in the "Teachers* Manual of Biology."
Some teachers may prefer the crayfish for beginning dissection (see § 310
in the "Teachers' Manual of Biology ") . For such work there are laboratory
directions in § 310 of the "Applied Biology," and mimeographed copies
could easily be made for Use of students.
100 INTRODUCTION TO BIOLOGY
Make an outline sketch of the frog as seen in profile and another one
of a boy as you imagine him walking on hands and knees, and then
write the four terms given above on your sketches so as to indicate
the parts of the body to which they are applied! Compare the right
and left sides of the frog. Remember that in the study of animal
biology right and left refer to the frog's body, not to your own. Hold
the tumbler so that the frog will sit with its head pointed away from
you, and your right will be the frog's right. If the animal were
lying on its back with head pointed away from you, as you look down
upon its ventral surface, would your right be right or left of the frog?
Are the two sides of the frog's body alike? Are the right and left
sides of your own body similar externally ?
At this point it will be well to have at hand a dead frog, one
either recently chloroformed or preserved for some time in formalin.
Examine the dead frog for all points not easily seen in the living
one kept in the glass tumbler.
Skin, — Note the color of the skin on the dorsal surface of the
frog's body. Compare with the color on the ventral surface. In a
later chapter it will be pointed out that many zoologists think that
these colors help to conceal the frog in the grass along ponds, among
water weeds, and in othelf places where frogs live. When you make
a trip into the country look for evidence that the color helps to con-
ceal the frog. The skin is covered with a slime or mucus. How
would this help the animal if an enemy tried to catch it?
Are there hairs on the frog's skin? Look with a hand-lens.
Compare with your own skin.
Sense-Organs. — (a) Examine eyes. Are there eyelids? Can
the frog close its eyes ? Look into a small mirror and compare your
own eyes with those of a frog. (6) Ears — the large, dark round
spots just behind the eyes are the membranes of the ears, stretched
like a drum-head over the cavities (internal ears), which will be
examined later. The frog has no projecting external ears such as
the human being has. Also in the human ear the ear-drum (tym-
panum) cannot be seen from the outside, because it lies deep in a
canal or tube which leads to the internal ear.
lAmhs. — Compare the anterior limbs with the posterior ones.
Are they alike? In the anterior limbs (fore legs) notice three
divisions: upper arm, extending from shoulder to elbow; fore-
arm, extending from elbow to wrist ; and the " hand." How many
** fingers " ? Compare the anterior limb with your own ; i.e., your
VSRTEB RATES
101
arm. What differences in the divisions do you notice? What
similarities ? In the posterior leg (hind limb or hind leg) of the frog
notice : the thick fleshy thigh, extending from the hip to the knee ;
the shank, extending from knee to ankle ; and the foot. How many
toes? Compare with number of " fingers." The shortest toe cor-
responds to the big toe of the human foot.
Notice the membrane (" web ") stretched between the toes of the
hind foot, fitting the foot as a paddle for swimming. When you
have an opportunity to
observe frogs swimming in
a large aquarium, or in the
clear water of a pond, com-
pare the uses of fore and
hind limbs. The fitting of
any structure or organ of an
animal or plant to a special
function is known as adaptor
Hon, Another adaptation of
the frog's hind legs is in the
great muscles of the thigh,
which fit it for jumping.
Mouth, — The opening is
the mouth, and inside is the
mouth-cavity (also called
buccal cavity, meaning
cheek cavity). Commonly
we speak of the human
mouth-cavity as mouth,
and the opening as lips ; but to be accurate " mouth '* should be
applied to the opening. Examine the mouth-cavity of a frog that
has been chloroformed. Are there teeth? Look for teeth on a
mounted skeleton of a frog. Notice' two small openings (nostrils)
on the dorsal side of the head near the upper lip of the frog's mouth.
Pass a shoemaker's bristle or a slender broom-straw into a nostril,
and note where the bristle comes out into the mouth-cavity. The
human nostrils have no such direct communication with the mouth-
cavity. With a needle make a hole in the frog's ear-membrane
back of the eyes, and then carefully push a bristle into the opening.
Open the mouth and find where the bristle comes out into the
mouth-cavity.
Fig. 35. Skeleton of frog. /, femur;
A, humerus ; «, scapula ; t, tibia ;
u, urostyle (end of backbone); », ver-
tebrsB.
102
INTRODUCTION TO BIOLOGY
Fig. 36.
tongue
insect.
Various positions taken by
of frog when catching an
{After Wiederaheim.)
To prove a similar connection between the human ear and the
back part of the mouth-cavity, close your nostrils with your hand,
then swallow once or twice and notice a feeling of pressure in your
ears, due to forcing air back into the middle part of the ears. This
explains why workmen are told to swallow air when entering the
compressed air chambers used
in tunneling under rivers. The
tubes connecting the middle
parts of the ears with the
mouth are known as Eusta-
chian tubes, named in honor of
Eustachius, professor of anat-
omy at Rome, who published
a description of the human
ear in 1754.
Examine the frog*s tonguey
and note that it is attached
at the forward end to the tip
of the lower jaw, while the
free end extends backward towards the throat. How does this
compare with the human tongue? In seizing an insect, the frog's
tongue is turned quickly forward out of the mouth, and then
quickly withdrawn. (See Fig. 36.)
89. Internal Structure of the Frog. — (Chiefly D) Look at
the mounted skeleton of a frog, compare with Fig. 35, and note the
position of the following bones : backbone (vertebral colunm) ;
skull; shoulder-girdle, which is a set of bones attaching the fore
legs to the body ; pelvis, a set of bones which attach the hind legs
to the body. Notice that the " ribs " are very short. Now turn
to the frog you have been studjdng, and feel the position of the
above-named bones through the skin.
Lay the frog on its back, !head pointing away from you. With
forceps lift the skin and with scissors carefully cut through it, along
the median ventral line, the whole length of the body. Carefully
separate the skin from the underlying parts, cutting the thread-like
connections, turn the flaps of skin outward to right and left, and pin
to the board or wax in the bottom of a dissecting-pan. Cover the
frog with water.
Notice the muscles of the body-wall of the abdomen, and the
bones connecting the fore limbs (shoulder-girdle).
VERTEBRATES
108
Again using scissors and forceps, carefully cut through the body-
wall in the median ventral line from the pelvis to the shoulder-girdle,
and then cut across the body (transversely) just posterior to the
girdle. Separate and spread out the two flaps of the body-wall,
and pin down to the dissecting-board. The cavity containing the
Fio. 37. Frog dissected from ventral surface. H, heart; Lg, lungs; L, liver;
Af, stomack; D, intestine; E, ovaries; B, bladder. {From Ecker.)
internal organs thus opened is the hody-cavity (ccelome). Now iden-
tify the organs exposed — liver, stomach, intestine, egg-organs or
ovaries (if the specimen is a female frog), comparing your specimen
with Fig. 37 in order to identify the organs.
Now cut out, with strong scissors, the ventral bones of the
shoulder-girdle (the teacher will demonstrate how this is best done).
104 INTRODUCTION TO BIOLOGY
As you lift up the bones, notice the heart lyimg beneath. The poste-
rior, conical, whitish part of the frog's heart is called ventricle (Fig.
37) ; it lies in a depression between two parts of the liver^ Anterior
to the ventricle are the thin-walled auricles (right and left), usually
found filled with dark blood in a dead frog. The ventricle is the part
of the heart which forces the blood from the heart into the blood-
tubes (blood-vessels) ; while the auricles are reservoirs for holding
blood coming back to the heart and collecting for the next " beat "
of the ventricle. The extremely thin membrane which incloses the
heart is the pericardium (meaning, around the heart).
(Z> or L) At the anterior end of the heart is a blood-tube (aorta)
which branches into two smaller tubes, each of which has several
smaller branches leading to the right and left * sides of the body.
These are the chief blood-tubes through which blood flows away
from the heart, and they are called arteries. The branches of the
two arteries lead to all parts of the body, and may be seen in many
organs. Connecting with the anterior end of the heart and on its
dorsal surface are three thin-walled tubes, which, in dead frogs, are
usually filled with dark purple blood. These are the blood-tubes
in which blood flows back to the heart, and they are called veins.
Later we shall see small branches of the three veiAs in various parts
of the body. In fact every organ which has ati artery in which
blood comes from the heart must have a vein for the return of
blood to the heart ; for the blood circulates from heart to arteries,
from arteries to smaller tubes called capillaries^ from these to veins,
and thence back to the heart.
(Z>) Capillaries, — Watch the flowing of blood through the
capillaries in the tail of a small tadpole allowed to lie on its side on
a wet plate of glass, or in the spread web of a frog's foot. Use low
power of microscope.
In addition to the system of blood-tubes carrying blood from the
heart, through the capillaries in all organs, and back again to the
heart, there are in all organs many small tubes which collect a
watery fluid called lymph. This is largely derived from the liquid
part of the blood, and it ultimately flows back into the blood-
The largest organ in the frog's body is the liver , a bi-lobed organ
♦ The student should remember that with the frog lying on its back the
right side of the frog will be on the observer's left. In all descriptions in this
book, right and left refer to the animal studied, not to the observer.
VERTEBRATES
105
lying behind and at the sides of the heart. Its color is reddish
brown in frogs not preserved in chemicals.
Ovaries. — In a fidl-grown female frog, masses of spherical black
and white eggs (ova) lie among the other organs (Fig. 37). These
ova are attached to, or are really united
to form, two organs (the ovaries or egg-
organs) which later will be found to be
fastened to the dorsal wall of the body-
cavity, one on the right and one on the
left of the median. line; i.e., bilaterally
symmetrical. With forceps, pull out the
masses of ova, taking care not to injure
other structures.
Alimentary Organs. — These are the
organs concerned with receiving food
and preparing it for the use of the frog's
body. The alimentary canals or food-
tube, extends through the body from the
anterior to the posterior extremity (Fig.
38). The mouth is the anterior opening ;
the posterior opening is called anus.
Turning the organs in the body-cavity,
but not cutting, examine the various
parts — stomach, intestine, etc. — of the
food-tube. The stomach lies dorsal to
the left lobe of the liver. The short
tube from stomach to throat is the esoph-
agus (gullet). The throat or pharynx
connects the esophagus with the mouth-
cavity. Carefully push a probe (such
as a small stick) down the throat into
the stomach. The tube extending from
the stomach backwards or posteriorly
is the intestine. Note that the first part
lies parallel to the stomach. The constriction between the stomach
and this first part of the intestine is the pylorus. The small intes-
tine is a slender and much convoluted tube. The large intestine
(or rectum) is a short, straight tube, of greater diameter than the
small intestine. The expanded end of the large intestine is called
cloaca. Tubes from the kidneys and reproductive organs open into
Fig. 38. Alimentary canal
of frog. Oe, esophagus:
Af, stomach; Du,D, amall
intestine ; Py, pylorus ;
Mz, spleen; R, large in-
testine ; HB, bladder ;
CI, cloaca. (From Ecker.)
106 INTRODUCTION TO BIOLOGY
the cloaca ; but this is not so in the highest animals. The opening
of the large intestine to the exterior is on the dorsal surface of the
body near the end of the backbone. In all animals this opening
is called anus.
Carefully cut the membrane (mesentery) which holds the alimen-
tary canal in place, cutting close along the canal, and pin the intes-
tine to one side and out of the body-cavity. Cut open the stomach
longitudinally. It may contain food, such as softened and disin-
tegrated pieces of worms, etc. ; the condition of the food suggests
that solid foods are dissolved in the stomach. Wash out the con-
tents, and notice the longitudinal folds which line the stomach and
increase the surface with which food comes into contact. Cut open
the intestine in several places. Do you find folds arranged as in the
stomach ? Notice that the food in the intestine is more liquid than
the food in the stomach. Also notice that by the time food has
passed along the posterior part of the intestine its bulk has been
greatly reduced — the stomach's capacity is much greater than that
of the intestine. Evidently the greater part of the food disappears
as it passes along the stomach and intestine. Where does it go?
In addition to the parts of the food-tube, the liver and the pancreas
must be considered alimentary organs, because they form or secrete
substances needed for preparing foods in the intestine.
The liver, whose position has been noticed, consists of two
lobes, the lobe on the left side of the body being larger and sub-
divided into two parts. Notice a small sac, the gaU-hladder, between
the right and left lobes. The greenish-colored fluid which fills the
gall-bladder is the bile, which is made by the liver and stored in the
gall-bladder until it is needed in the intestine. A bile-duct (very
small and difficult to see) leads from the gall-bladder to the intestine.
(D) The pancreas lies between the stomach and intestine. It
is an irregularly lobed mass of light color in fresh specimens. It
secretes a fluid (^pancreatic juice) used in digestion of foods. Many of
its small ducts open into the bile-duct, which extends along the pan-
creas on the way from the gall-bladder to the intestine.
Cut the attachment of the liver, esophagus, and intestine and
remove these organs, thus exposing other organs nearer the dorsal
part of the body-cavity.
(Z>) The lungs are two thin-walled sacs, which, before removal of
the liver, were dorsal to that organ. Cut through the articulation of
the jaws so as to allow the mouth to open widely, and demonstrate
VSRTEBRATES 107
the slit-like glottis in the pharynx just ventral to the opening of the
esopha^s. The tube into which the glottis opens is the windpipe
or trachea, which has two branches leading to the lungs. Insert the
end of a small tube (blow-pipe) into the glottis and blow in air to
inflate the lungs ; or, using a rubber-bulbed pipette, fill them with
water.
(L) Watch a living frog in a glass jar, and observe how the floor
of the mouth (" throat ") moves up and down at regular intervals.
Count the movements in one minute. The frog cannot breathe as
we do but practically " swallows "air, forcing it down into the lungs
by moving the floor of the mouth up while the mouth and nostrils
are closed. Many persons breathe while keeping the mouth open,
but a frog cannot breathe when the mouth is held open. Not every
movement of the throat forces air down into the lungs; most of
the movements simply pump air into and out of the mouth-cavity
through the nostrils. But watch carefully, and occasionally the
sides (flanks) of the body back of the shoidders will be seen to expand
greatly when the frog seems to take a big " swallow *' of air; this
expansion indicates that air has been forced down into the lungs.
Then, after a time, the muscular walls of the body help the elastic
lungs expel the air, forcing it up into the mouth-cavity, where
it is gradually mixed with fresh air pumped in through the
nostrils.
(D) The kidneys are flat bodies of a dark-red color. They are
attached to the dorsal body-wall in the posterior portion of the body-
cavity, one on each side of the backbone. The ureters are the ducts
or tubes of the kidneys which lead to the terminal part of the large
intestine {cloaca) ; they are very small and difficult to demonstrate.
The kidneys extract water and certain other substances from the
blood which flows through them, and then pass these substances to
the exterior through the ureters and cloaca. Before these sub-
stances from the kidneys are eliminated, they may be stored for a
time in a sac known as the bladder, which lies on the ventral side of
the cloaca, and opens into it. This structure is usually found as a
collapsed membrane if one looks carefully before cutting out the
large intestine.
The spleen is a small, oval, red body attached to the mesentery
near the large intestine.
Reproductive Organs. — (D or L) Examine these organs in speci-
mens of each sex. In the male frog the spermaries are a pair of yellow
108 INTRODUCTION TO BIOLOGY
ovoid bodies attached ventrally to the kidneys. Their ducts
lead into the kidneys, and there connect with little tubes leading:
to the ureters, and thence through the cloaca to the exterior. These
organs produce sperm-^eUs which fertilize ova or egg-celhy as described
later, in Chapter XII.
In the female, the ovaries are attached at about the same place.
The oviducts are a pair of long, much-coiled, white-colored tubes,
lying close to the dorsal wall of the body-cavity and at the sides of
the kidneys. , Posteriorly they open into the terminal part (cloaca)
of the intestine, while anteriorly they have funnel-shaped openings
into the body-cavity dorsal to the liver. There is no direct connec-
tion between the ovary and the oviduct, but the ova, when ma-
ture, fall into the body-cavity, pass through the above-mentioned
openings into the oviducts, and then to the exterior through the
cloaca. (The use of the reproductive organs will be described in
Chapter XII.)
The fat-bodies are tufts of bright-yellow masses attached to the
dorsal side of the body-cavity behind the liver. The fat is food
stored for use in the early spring. Such fat-bodies are found only
in frogs and their near relatives, but other animals store fat in
various organs.
Nervous System. — (D or L) Remove all the organs which have
been studied and cut away the floor of the mouth. Notice (1) the
skully which is covered on its ventral surface by the roof of the mouth,
and (2) the backbone or vertebral column. The skull contains the
brain, and the backbone is a tube which incloses the spinal cord.
Looking at the body-cavity side (i.e., ventral), notice the large
nerves which extend from the vertebral column to the fore and
hind limbs; also some small nerves extending out to the body-
wall of the back. Examine brains which have been hardened
by chemicals and then removed, and also observe a specimen
of a frog dissected from the dorsal side to show brain and spinal
cord.
Skeleton. — The chief bones of the frog can be identified by com-
paring a skeleton with Fig. 35.
The muscles of the frog*s legs should be examined as to their
attachments to the bones. Also, note how shortening of muscles
woidd affect the movements of bones to -which they are attached.
There are other muscles in the body-wall, and in the walls of stomach
intestine, and blood-tubes.
VERTEBRATES 109
90. Organs of Frog. — Summarizing, we have found the
frog to be made up of parts or organs as follows : —
Alimentary organs : mouth, mouth-cavity, pharynx,
esophagus, stomach, small intestine, large intestine (includ-
ing cloaca), liver, and pancreas.
Breathing organs : nostrils, mouth-cavity, pharynx,
trachea, lungs, and skin.
Circulation organs : heart, arteries, veins, capillaries,
lymph-vessels, and spleen.
Excretory organs : kidneys, ureters, bladder, cloaca, lungs,
and skin. ^
Nervous organs : brain, spinal cord, nerves, eye, ear, and
nose.
Supporting organs : skeleton (bones and cartilages).
Muscle organs : muscles of body-wall, of limbs (for loco-
motion), and of internal organs (heart, stomach, etc.).
Reproductive organs : ovaries, spermaries, ducts of ovaries
and spermaries, and fat-bodies (peculiar to frog and its rela-
tives or allies).
II. THE TISSUES OF COMPLEX ANIMALS: MICRO-
SCOPIC STRUCTURE
91. Structure of Organs. — The bodies of complex ani-
mals, such as insects, crayfish, snails, fishes, and frogs, are
composed of many parts which we call organs, examples of
which are skin, liver, stomach, heart. We shall now examine
more carefully the structure of some organs.
(D) Examine a frog*s leg. First on the outside, notice the skin.
Remove the skin, and the muscles are brought into view. Compare
a piece of skin with a piece of muscle. Is there any difference ? In
separating the skin from the muscle, or the muscles from each other,
notice fine but strong threads binding them or connecting them.
Notice bright, glistening bands (called tendons) which unite muscles
110 INTRODUCTION TO BIOLOGY
to bones. Next, examine the nerves which lie between the large
muscles. Scrape the muscle from the bone and attempt to cut into
it about the middle and also at the rounded ends. Is there any
difference?
In thus examining the leg of any animal we find that it
is composed of several distinct kinds of building materials.
These are tissues. The surface of the skin is an example of
protective tissue which is called epithelium or epithelial
tissue. The strong threads binding skin to muscle, muscles
to each other, and (in form of tendons) muscles to bones, is
known as connective tissue. The muscles contain rmuscular
tissue^ and the nerves have nervous tissue. The hard part of
bones is bony tissue, and the softer tissue at the ends of
bones is called cartilage.
(L) Examine your own hand. Skin or epithelial tissue covers
and protects it. Pull upon the skin, and it is found to be bound
(by connective tissue) to the underlying muscles, which you can
feel and move. You can also feel bones. Lastly, you have the
sense of touch or feeling in the hand ; this indicates the presence of
nerves. Name the tissues which you find in your hand. Do you
find any which were not named above ?
The tissues that have just been examined are the kinds
of building materials which form not only an animal's legs,
but also its whole body. The same kinds are also in our own
bodies. If we were to examine any organ in a frog or in
the human body, we should find it made up of two or more
of the tissues. For example, the heart is largely composed
of muscular tissue, but it has ;nerves, connective tissue, and
epithelium ; and the stomach has epithelium, muscles, nerves,
and connective tissue. Any complex animal's body is made
up of many kinds of materials or tissues which have different
appearances and serve different purposes. And just as the
materials (iron, stone, brick, wood, etc.) used in building
houses may be put together in various combinations so as
VERTEBRATES 111
to form many different kinds- of buildings for different pur-
poses, so the few kinds of building materials or tissues of an
animal's body are united to form organs which are quite
different in appearance and purpose. An animaPs heart
does not resemble a leg muscle and their purposes and work
are different, but they are chiefly composed of the same kind
of tissue (muscular), because muscular activity is needed
for movement in legs and in the heart. In like manner, we
find epitheliiun wherever there is a surface, inside or outside,
to be covered ; cartilage wherever flexibility combined with
considerable rigidity (e,g,j at ends of bones) is needed;
connective tissue wherever other tissues must be joined
together; bones for supporting framework; and nervous
tissue in all places where nervous activity (feeling, sensation,
control, etc.) is needed. Each tissue has its peculiar purpose,
just as wood, bricks, and iron have their own purposes.
Briefly, the purposes or functions of the tissues are as follows :
epithelium for covering, connective tissue for uniting, bone for
rigid support, cartilage for flexible support, muscular tissue
for contraction and movement, and nervous tissue for feel-
ing and control.
So far we have been studying the larger structure of ani-
mals as seen by the unaided eye. We have been able to
locate the various organs and to learn something about their
general form or position; but concerning the structure of
the organs themselves our unaided eyes have been able to
discover only the tissues. It is now necessary to make use of
the microscope in order to see the minute structure of the
tissues which we find in organs.
(L) If Chapter III has not been studied, a lesson on the use of
the compound microscope sh<5uld be introduced at this point.
92. Cells. — (D) Mount a small piece of the outer skin (epithe-
lium) of the frog in a drop of water on a glass slide and covei* with
112
INTRODUCTION TO BIOLOGY
a cover-glass. Examine this with a compound* microscope (magni-
fication 50 to 100). The epithelium is seen to be composed of small
(usually hexagonal) blocks, called
ceUsy set side by side like bricks in
a wall or pavement (see Fig. 39).
A small spherical mass (called
nucleus) may be seen near the
center of each cell, and in most of
the cells the nuclei may be brightly
stained by dipping a piece of epithe-
lium into a dye such as eosin solu-
tion (red ink), then into water, and
then mounting on a glass slide for
microscopic examination. In the
same slide notice that the cells are
closely joined together ; in fact,
there is be-
tween them a
cement sub-
stance, which
can be dis-
solved with some chemicals, and the cells are
then easily separated.
Cells. — We have chosen the frog's
epithelium as a convenient introduction
to cells. All other animal and plant
tissues which biologists have examined ^iq. 4o. Upper fig-
microscopically have been found to have
cells ; and so the cells are regarded as the
units of which the bodies of organisms
are composed. Various forms of cells
are shown in Figs. 39, 41, 42, 43, and 49.
The word cell commonly means a cavity,
and it was originally applied to certain
plant cells (e,g,j in cork and elder-pith which have cavities
in their substance) ; but it is now known that most animal
Fig. 39. Diagram of cells from
frog's skin. cw, cell-wall ;
cb, cell-body; n, membrane of
nucleus; black spots in nucleus
are chromatin. (From Ver-
wom.)
ure shows that there
are many layers of
cells in epidermis,
those of Fig. 39 being
at the surface. The
lower diagrams rep-
resent two cells in
surface and edge
views.
VSRTEBBATES
113
Fig. 41. Unstriated
muscle cells, n, nu-
cleus.
cells and many plant cells do not have cavities. Never-
theless, the word cell has become firmly fixed in biological
language, and so we must use it as the
scientific word for the elements or units
of animal or plant tissues. The spherical
structure seen near the center of each cell
of the frog's skin is the most conmion
form of nucleus ; but in some cells of
other animals the nuclei are in various
forms — ribbon-like, like a string of beads,
or even scattered in fragments in the sub-
stance of the cell. Apparently every
living cell has a nucleus of some one of
these form^. The sub-
stance of a nucleus is
called nuclear material. Careful experiments
have proved that a cell cannot continue to
live without a nucleus. Organisms grow by
multiplication of cells, and the nucleus plays
an important part in that process of cell-
division. Notice the similarity of the
animal and plant cells in Figs. 46, 47. The
boundary of a cell is called cell-wall or celU
Fig. 42. Parts of membrane; it is sometimes exceedingly^ deli-
^ecdiTfrom ^ate, and sometimes very thick and hard,
as in certain plant cells. The substance
between the cell-wall and the nucleus is
called the cell-body. Cells are composed of
tween c^^ ^^ cell-substance, part of which is living sub-
stance and part is stored food and other
lifeless materials to be considered later.
93. Inter-cellular Substance. — In some animal tissues
all the substance does not lie within cell-walls; some of it
a leg, showing
many nuclei (n)
in each cell, and
blood - capillaries
114
INTRODUCTION TO BIOLOGY
is between cells or inter-cellular. In the frog's skin and
muscles there is a cement substance between cells which
holds them together. This is called inter-cellular substance,
and is formed or secreted by the cells. In an epithelial
tissue, such as the frog's outer skin, there is comparatively
little inter-cellular substance. In some tissues there is a
large proportion of inter-cellular sub-
stance, and an example will now be
examined.
(D) With a sharp knife or razor cut
very thin slices of cartilage or *' gristle"
from the joint end of a bone procured at
a butcher's shop, or from a frog's bone.
Mount these slices in a drop of water on
a sUde. Examine with . a microscope.
Notice that the greater part of the tissue
is a bluish-white substance in which the
cells lie. The distance of one cell from
another is often more than the diameter
Was this so in the epithelium of the skin?
Fig. 43. Section of bone.
c, cavities containing bone
cells ; h, canal ; I, layers
or lamellae of bone se-
creted by bone cells.
of the cells (see Fig. 44).
It is evident that the greater
bulk of cartilage is not cells,
but the substance between
the cells (i.e., inter-cellular).
A section of. dry bone is
similar to cartilage in that
there are numerous small cav-
ities in which originally were
located the cells that formed
the hard bony substance,
which is entirely inter-cellular
(Fig. 43).
'^
6'
Fig. 44. Cartilage from end of femur of
frog, c, cartilage cells; m, matrix of
inter-cellular substance formed by the
cells; C.8, empty cell spaces. {From
Parker.)
and cartilage and all the organs of the frog's body, everywhere
we should find cells and inter-cellular substance. We may
If we were to examine
all the tissues besides bone
VERTEBRATES
115
therefore make the general statement that the
body of the frog is composed of cells and inter-
cellular substance. But the latter is formed or
secreted by the cells, and hence the cells are
a
Fig. 46. Epidermis of salamander tadpole.
Three cells undergoing division. {From
Wils&n,)
Fig. 45. Con-
nective tis-
sue, c, cells;
w, white fi-
bers; c, elas-
tic.
the primary units composing the body. These statements
are also true of all higher animals and of the human body.
94. Life-Activities in
Cells : Protoplasm. —
Since the bodies of all
complex animals are
composed primarily of
cells, we are led to infer
that the life-activities
(§ 29) are located in the
individual cells. For
example, the shortening
of a muscle when it con- ^'""'f- Epidermis from onion bulb Seven
cells are in vanous stages of divisioa.
tracts and moves is the {From wuson.)
116 INTRODUCTION TO BIOLOGY
result of the combined shortening of the thousands of cells
of which the muscle is composed. It appears, then, that the
living active substance is located in the cells of the body.
The technical word for living matter is protoplasm. It is
not known how much of the cell-substance is living ; but it
is certain that the protoplasm is the basic or essential sub-
stance in both nucleus and cell-body. The " life " of an
animal, then, is not limited to any one organ, such as the
heart, the brain, or the lungs ; rather it is located in every
living cell of the body.
In order to understand better that protoplasm is living
and active, the microscope may be used to examine leaflets
of certain aquatic plants in which the cells are so transparent
that we can see the protoplasm moving (flowing) aroimd in-
side the cell.
(D) Observe movement (streaming) of protoplasm in leaflets of
Elodea, Nitella, or Chara — all of which are widely distributed
aquatic plants. Mount leaflets in fresh water, and select trans-
parent places for study with low power of microscope. In Elodea
the bodies which contain the green-colored matter (chlorophyll)
move with the protoplasm. Sometimes the movements are checked
temporarily by the jarring involved in mounting, and it is necessary
to let the slide stand for a half-hour or more before the protoplasm
shows its most rapid motion.
III. THE WORK OF THE ORGANS OF THE FROG : IN-
TRODUCTION TO ANIMAL PHYSIOLOGY*
96. Need of Food. — We shall now consider the work of
the organs of the frog's body, and first we may inquire, Why
does it need food? Some one may answer, "To keep its
body alive," but that is not a scientific answer. In study-
* The statements made in this lesson may be directly applied to any
higher animal.
VERTEBRATES 117
ing science we want to kxiow why and how food is used by
animals so as to " keep going " the life-processes.
Waste, — A living animal is active in a number of ways,
such as moving and breathing. These activities result in a
loss of weight in the body-substance. In other words, the
living body of the frog behaves like a machine in that all
activity leads to wearing out. A new steam-engine begins
to wear out as soon as it is put into motion, and at every
working part, particle by particle, it is worn away. Like-
wise, the frog is a machine in which wearing out or wasting
is continually occurring in every organ of the body; for
though not always visible, every living cell of its body is
continually working.
Repair and Growth. — Now, it is evident that this lost sub-
stance must be replaced by new substance, or the body
will soon wear out and die. In other words, the waste
which is always occurring in the living animal must be made
good by the processes of repair. The materials for this repair
are supplied by the animal's food. And in addition to ma-
terials for repair, food also supplies the materials required
for growth, which means increase in number of cells.
What would be the effect on the weight of the body if
the wasting processes were more rapid than those of repair?
What are the conditions with respect to the rate of waste
and repair in a young frog that is growing rapidly?
A scientific answer, then, to our question, " Why does the
frog need food? '' is that food (1) furnishes the new sub-
stances necessary for assimilation to replace the body-
substances which have been wasted owing to activity ; and
(2) in addition to repairing, food may furnish the materials for
growth. Especially do young animals need food for growth.
Energy, — The question " Why does the frog need food? ''
may be considered from another point of view. The
118 INTRODUCTION TO BIOLOGY
activities - — breathing, eating, moving, etc. — indicate that
the animal is continually doing work or using energy. Energy
is the capacity for doing work. For example, the energy of
the coal may move the steam-engine, the engine may run a
dj^amo and produce electrical energy, and the electricity
may run a trolley-car.
Now, it has been discovered that energy cannot be created
by any machine, plant, or animal ; but energy may be trans-
formed. Thus the energy of the coal is transformed into
heat energy and the mechanical energy of the engine, the
mechanical energy is transformed into electrical energy, and
this into heat energy (electric heater), light energy (electric
lamp), or mechanical energy (electric motor). A steam-
engine does work and expends energy; but it derives its
power of doing work (that is, its energy) from the coal burned
in the furnace. Likewise the frog, being unable to create
energy within itself, requires food as a source of the energy
which is to be used in the life-activities. The food of the
frog corresponds to the coal of the engine, in that each sup-
plies the energy for its respective machine.
The value of the food of the frog or of the coal depends
upon the amount of stored or potential energy which each
contains. Thus a ton of one kind of coal may run a certain
large engine eight hours, while another quality from another
mine may make the engine work just as hard for ten hours.
Likewise, the amount of energy in foods is highly variable.
Chemists have methods of burning samples of foods and
fuels and determining how much energy they are capable
of furnishing ; that is, how much stored or potential energy
they contain. Such tests of energy value are especially
interesting and important in connection with human food,
and will be studied later.
In addition to supplying energy for the life-activities, we.
VERTEBRATES 119
should note that the food 'of the frog also supplies the ma-
terials for repair and growth of the frog's body, while the
coal only furnishes energy and is powerless to repair the con-
tinual wear of the engine. Here is the great difference be-
tween SL living machine, such as the frog's body, and the life-
less engine. Only the living machine, the animal's body, has
the power of using food in order to repair the parts wasted
by activity. When the animal's body grows old, it loses the
power of using food to make repairs, and some of its essential
parts wear out and stop the activity of the living machine.
Summary : Uses of Food, — We can now give a complete
answer to the question, "Why does the frog need food? "
by saying (1) food supplies energy, (2) food supplies the
materials for repairing the waste of the living animal body,
and (3) food suppHes the materials for increase in size
(growth), especially in the case of young, growing individuals.
These three statements are true of all living things.
96. Changes in Food. — Having learned that food sup-
plies materials for repair, growth, and energy of the frog's
body, we now turn to trace it as it is taken into the body and
undergoes the changes which take place internally when it
supplies energy, builds up the body, and repairs the wasted
body-substances.
In an earlier lesson we found that an animal body con-
tains water, carbon, gaseous substances, and mineral sub-
stances. In order that food may serve for making new
body-substance for growth or repair it must contain all
these ; in fact, all the chemical elements found in the body.
Dissolved Food. — The food which is taken into the mouth
of the frog and thence passes through the esophagus into
the stomach is largely solid, such as worms and insects.
These must be reduced to a liquid condition before they can
be absorbed through the lining of stomach and intestine into
120 INTRODUCTION TO BIOLOGY
the blood. Solid food retained in the stomach cannot fulfill
its purpose of supplying the energy and materials for repair,
for the reason that energy is being expended and repair is
necessary, not only in the cells composing the stomach, but
also in all living cells of all the organs of the body, even in
the remotest parts, such as the fingers and toes. It is evi-
dent that solid food cannot be distributed so widely; but
a solution (like sugar dissolved in water) can be absorbed by
the lining of the stomach into tubes (blood-tubes) through
which it can flow to distant organs and then " soak " into
the cells. Hence we see the necessity of rendering solid
food soluble in the water which is taken with food.
(D) A drop of milk, when examined with the microscope, is
found to contain numerous fat droplets. Shake in a small bottle
a few drops of oil, some dissolved soap and some water ; and ex-
amine microscopically a drop of the milky mixture. This and milk
are examples of emulsions. A drop of water containing some starch
shows the small particles mixed with the water. Examining with
the microscope a drop of water in which sugar or salt has been dis-
solved, no particles of these substances are evident.
Milk and the mixture of water and starch are examples
of fluids or liquids, but they are not solutions. The particles
of starch or fat are not dissolved (i.e., not soluble) in the
water. On the other hand, the water-and-sugar mixture is a
liquid or fluid, and it is also a solution. It is clear that all
liquids are not solutions. This distinction should be kept
in mind for use in connection with the changes which foods
undergo in the alimentary organs, for in them foods are
chiefly prepared for absorption by being made into a solu-
tion in water. Even liquid foods like milk and soup must be
dissolved so that no solid particles can be seen when they
are examined with a microscope.
97. Digestion. — Some solid foods, such as sugar and
salt, readily dissolve (are soluble) in water ; but most of the
VERTEBRATES
121
frog's food consists of meat and other things which have to
be acted upon by certain substances before they will dissolve
in the water which is taken into the food-tube. To this
process of changing fopds and causing them to dissolve the
term digestion is given. Definition : Digestion is the prepa-
ration of foods for absorption by the dells. It is partly a chang-
ing of foods so that they dissolve in water.
In order to understand how a chemical change may make
an insoluble substance capable of dissolving (soluble) in
water, try the following experiment : —
(D) Place a very small piece of marble or limestone (or a piece
of zinc) in water. Does it dissolve ? Pour some dilute hydrochloric
acid into the water, and repeat from
time to time until the limestone (or
piece of zinc) becomes dissolved in the
water.
From such experiments we learn
that some substances which are not
soluble in water will become soluble
after a chemical change has been
produced by an additional sub-
stance. This is just what happens
in the alimentary canal of the frog.
Small pocket-like tubes on the inside
wall or lining of the stomach and
intestine (called respectively gastric,
Fig. 48, and intestinal glands), as
well as the liver and pancreas, form
or secrete peculiar fluids (called secre-
tions) , which are poured into the
stomach and intestine. These, coming into contact with
foods, change them so that they are dissolved in water taken
with the food. When we say that the secretions of the
Fig. 48. lining membrane
of frog's stomach. (7, gastric
gland composed of epithe-
. lium ; c, connective tissue ;
m, muscle coat of stomach.
{From Ecker.)
122 INTRODUCTION TO BIOLOGY
gastric and intestinal glands and of the liver and pancreas
digest the various kinds of foods which the frog eats, we
simply mean that the foods are dissolved and prepared so
that they can be absorbed through the lining of the stomach
and intestine into the blood-tubes. The nature of these
digestive secretions and of their action on the various kinds
of foods will be taken up more carefully in close connection
with similar processes which occur in the human body. For
our present purpose it is sufficient to point out that digestion
is preparing foods for absorption through the cells which
line stomach and intestine.
98. Distribution of Digested Foods. — We have noted
reasons why digested foods must be distributed to all parts
of the body (see § 96). We shall now consider the means
by which this is accomplished.
Absorption. — If the frog were a small animal with all its
tissues near the digestive cavity of the alimentary canal,
the dissolved food could be absorbed through the walls of
the stomach and intestine into the surrounding organs.
This is the case in some very small animals of simple struc-
ture, such as Hydra. We can illustrate the absorption of
digested (liquid) food in such a simple animal by the follow-
ing experiment : —
(D) Take four sheets of blotting-paper (or filter-paper) about 4
by 10 inches in size, place them together, then roll them into a cone,
and fasten with a pin. Dissolve common salt in water to make a
strong solution, and pour into the cavity of the paper cone. Notice
that the salt solution is absorbed first by the inner layer of paper,
and then in succession by the outer layers, each layer absorbing
from the next one nearer the center. Now unroll the sheets, mark
the inner one No. 1 and the outer No. 4, and dry them. When dry,
notice the crystals of salt on sheet No. 4, proving that the salt in
solution has been absorbed from sheet No. 1 to 2, from 2 to 3, and
from 3 to 4.
VERTEBRATES 123
Some simple animals have a cylindrical body made up of
several layers of cells which are in close contact similar to
that of the layers of paper in the above experiment. (See
Fig, 54, of Hydra.) In such simple animals the cavity in the
center of the body is the digestive cavity in which food is
prepared for absorption. The cells which line this cavity
are in contact with dissolved food and can absorb directly,
just as did sheet No. 1 in the above experiment; and the
cells which form the outer layers must absorb from the inner
ones, just as in the above experiment the outer layers of
paper absorbed from those nearer the center.
99. Need of Blood for Distributing Food. — Absorption
directly from the digestive cavity of the stomach or intestine
would be possible only in a very simple animal. If in the
above experiment we had used twenty or thirty sheets of
paper, we should have found that very little salt solution
would soak through to the outermost layer ; and likewise in
any animal with many layers of cells the outermost cells
would get insufficient food if they had to depend upon ab-
sorption by layers, as in the case of the experiment with
paper. Obviously, absorption directly from the stomach
and intestine would not be possible in an animal like the
frog, in which, as we have already learned by dissection,
many organs are at some distance from the digestive cavities
of the stomach and intestine. Clearly, then, there must be
some method of transporting digested food from the diges-
tive organs (stomach and intestine) to all parts of the body.
This is one work of the blood, which, as we have already found,
flows in a set of tubes, leading to all parts of the body.
The details of the way in which blood carries dissolved food
from the stomach and intestine to all parts of the body will
be studied in a later lesson in connection with the same
process in the human body. The important point fpr our
124 INTRODUCTION TO BIOLOGY
present study is that the frog needs blood and a mechanism
(pumping heart) for moving it, in order to carry absorbed
food from the stomach and intestine to all parts of the body ;
that is, to every living cell where it can be used as a supply
of energy and as material for repair and growth. We shall
later find two other important reasons why the frog needs
blood.
100. Changes in Living Cells : Oxidation. — We turn now
to the changes which occur in the living cells everywhere
in the body of the frog. As has already been stated (§ 95),
the continuous activity of the living animal causes a wearing
out of the body-substance ; that is to say, of the particles of
cell-substance in the tissues composing the body. The wear-
ing out changes are largely due to the chemical union of
oxygen with the cell-substances, producing a kind of slow
combustion or oxidation. We commonly think of combus-
tion as the rapid burning of substances in the open air ; but
essentially the same kind of change may take place slowly.
For example, a piece of iron wire can be quickly burned
(oxidized) in a jar of pure oxygen. But in the oxygen of the
air, diluted as it is with nitrogen, oxidation of iron is the very
slow process of " rusting." Another example : A piece of
magnesium wire may be burned quickly at a high tempera-
ture (§ 9) ; but if we place a piece of magnesium wire in
water kept at the ordinary temperature, it will slowly oxidize
(form a white powder on the surface). Now, the slow oxida-
tion of the magnesium wire in water, or the familiar rusting
of iron, illustrates the slow chemical changes which are con-
tinually taking place in every living cell of animals and plants.
We emphasize the word " continually," for oxidation of the
cell-substances is invariably associated with the activities
which we call '^ Uving " ; and as long as there is life in a cell,
oxidation is going on and particle by particle the cell-sub-
VERTEBRATES 125
stance and the food brought by the blood are being burned ;
that is, they are being combined with oxygen to form new
substances.
Heat. — This slow burning (oxidation) results in heat,
and this is how the human body is kept warm. In frogs
and other lower animals the heat thus generated is lost
rapidly, because the surface of the body is not covered with
hair or other structures to prevent loss of heat, and so the
frog is never perceptibly warmer than the water which
touches its skin. We call it " cold-blooded," because it is
usually colder than the human body.
101. Oxygen Required. — We have just learned that oxi-
dation is a process necessary to the life of the cells in the
frog's body. Oxidation requires oxygen; and therefore
there must be in the cells of the frog a continual supply of
that gas. In some very small aquatic animals oxygen is
absorbed from the surrounding water by all the cells, and
there is no need of any special organs for supplying oxygen.
But in an animal as large as a frog, the cells of the internal
organs are so far from the external air. and water that some
method of distributing oxygen is required, just as it is neces-
sary that digested food be transported to cells which are
far away from the digestive organs. The distribution of
oxygen is accomplished by the blood, which absorbs oxygen
from the air or water external to the body and then carries
it to the internal cells. When the frog is under water, oxygen
(a small quantity of O is always mixed with or dissolved in
water exposed to the air) is absorbed by the blood flowing
through the blood-capillaries in the skin; but when the
animal is on land, oxygen is absorbed directly from the air,
partly by the blood flowing in the capillaries in the skin and
also by the blood flowing in the capillaries of the lungs. In
our own bodies the blood absorbs most of the necessary
126 INTRODUCTION TO BIOLOGY
oxygen in the lungs. In fishes the blood flowing through
the capillaries in the delicate membranes of the gills absorbs
oxygen from the water just as the frog's skin absorbs some
oxygen when the animal is under water.*
(D) Examine specimens of frogs* lungs with injected blood-
vessels, frogs* skin with injected blood-vessels, gills of a fish injected
to show blood-vessels. (These may be obtained by injecting with a
hypodermic syringe or sharp-pointed pipette some colored mixture
such as starch and carmine in water, into the large arteries of a frog
which has been killed with anaesthetics.)
102. Excretions. — It has been stated that the cell-sub-
stance (including foods absorbed by cells) is continually
being oxidized in every living cell of the frog's body. This
chemical change produces a number of substances which are
of no further use in the cells of the body ; in fact, they would
be poisonous if allowed to accumulate. These waste sub-
stances are called excretions.
We have already noted (§§ 11-15) that animal substance
may be analyzed into water, gas, carbon, and mineral
matters ; and also that the gas and carbon may be burned.
When food and cell-substance is oxidized in the living body,
the chief excretions produced are of four kinds : water, carbon
dioxide (a gas formed by burning the carbon of cell-substance),
mineral substances, and peculiar substances containing nitro-
gen and called nitrogenous excretions. Three of these — water,
carbon dioxide, and mineral matter — are easily shown to be
present when animal matter is heated and burned in a pipe or
tube (§ 13). Also it could be proved by a careful test that one
of the gases given off when the meat is heated is ammonia ;
and this gas contains nitrogen, which is a constituent of the
nitrogenous excretions formed when cell-substance oxidizes in
* To Teachers: Injected specimens suggested in § 48 of the "Applied
Biology" are a useful part of the permanent equipment of the laboratory.
VEETEBRATE8 127
a living cell. It is true, then, that all the substances found
when we analyze animal matter by heating and burning are
also present in the excretions formed by oxidation which takes
place at the temperature of the frog's body. The chief dif-
ference is that oxidation in the test-tube is rapid and at high
temperature, while in the frog's body it is slow and at low
temperature.
103. Removal of Excretions. — The excretions formed in
all the living cells of the frog's body are poisonous if allowed
to accumulate. Hence they must be eUminated from the
body. Kidneys, skin, and lungs are the organs of excretion
or excretory organs. Dissection of the frog showed that
many cells are at such a distance from these organs of ex-
cretion that the poisonous substances cannot be absorbed
directly by these organs, thus making it necessary that the
blood should absorb excretions from the cells, and, flowing
to the excretory organs, give up these excretions to be elim-
inated from the body of the animal. In the frog, the
carbon dioxide is carried by the blood from the cells, where
it is formed, to the skin and lungs, where it is given ofif to
the air or to water. The nitrogenous excretions are first
absorbed from the cells by the blood and then carried to
the kidneys, where, along with water, these excretions are
removed from the blood, passed into the ducts (ureters),
and thence to the exterior. In some simple aquatic animals,
the excretions are absorbed directly from the cells by the
water in which the animal lives; and just as in th^case of
supplying food and oxygen to these animals (§§ 99, 101),
there is no need of blood as a carrier of excretions.
104. Respiration. — This is a term which has long been
used in physiology as almost a synonym for the popular word
" breathing." It includes two of the processes which have
been described ; namely, taking in oxygen and giving out or
128 INTRODUCTION TO BIOLOGY
excreting carbon dioxide. In most animals the two processes
go on at the same time and in the same organs. Air taken
into the lungs supplies oxygen to the blood and at the same
time absorbs carbon dioxide from blood circulating in the
lungs; and hence the lungs are often called respiratory
organs. Keep in mind for use in future lessons that respira-
tion includes (1) supplying oxygen to cells, and (2) excreting
carbon dioxide from cells.
106. Summary of Functions of the Blood. — We have
found that blood and a mechanism for its circulation are
necessary in the frog for communication between the living
cells and certain organs which communicate with the ex-
ternal world. As we have seen, all cells must have a supply
of food and oxygen and must get rid of the substances
(excretions) resulting from the oxidation of cell-substances.
Some small animals have all their cells near the places where
food and oxygen must be absorbed and excretions eliminated ;
and for this reason such small animals need no blood-system.
But in the frog, and in all except the simplest animals, there
are cells at some distance from the organs which supply
oxygen and food, and also far from those which eliminate
excretions ; and in all such animals the blood acts as a trans-
porting medium which (1) carries food and oxygen to the
cells from the organs (lungs and digestive organs) which
obtain these substances directly from the exterior, and
(2) carries excretions from the cells to the organs (lungs,
skin, oik kidneys) which pass them out of the body. In ani-
mals higher than frogs and reptiles the blood is also important
in distributing heat and maintaining uniform temperaturiB.
106. Nutrition. Fundamental Processes. — We have now
briefly traced food from its entrance into the frog's body
through the changes of digestion, absorption, and distribu-
tion to the living cells. These cells are active living ma-
VERTEBRATES 129
chines requiring food (1) for repairing their waste and for
growth, and (2) for oxidizing to give the energy which is ex-
pended in the activities of the body. Sooner or later most of
the materials entering the cells as food become combined with
oxygen, and the resulting substances are excretions of no
further use to the living cells.
All the changes which food and oxygen undergo, begin-
ning with their reception into the body and ending with their
elimination in the form of excretions, involve the processes
of digestion, absorption, circulation, respiration, changes
within cells (metabolism) f and excretion. It is important to
remember that all these processes are fundamental, for one
is just as necessary as another. All the processes ■^— taking
of food, digesting of food, its absorption by the blood, its
transportation to the cells, its absorption by the cells, its
use by the cells, the supplying of oxygen, and the removal
of excretions — all these processes are linked together, as it
were, in a chain, and each process must play its part in the
life of the body.
107. Need of Organs Working Together : Co5rdination. —
All the organs concerned in the processes named in the para-
graph above must work together, for if any organ fails in
the proper performance of its work, the result is that the
working of all the organs of the body is afifected. For ex-
ample, if the heart beats slower, the blood flows slower, and
consequently the supply of food and oxygen to the cells and
the removal of excretions will be lessened. We know that
if the heart stops beating, or the lungs cease acting, animals
die at once ; and the reason is that the cells of the body fail
to get their necessary food and oxygen and the poisonous
excretions are allowed to accumulate. We see then how
necessary it is that all the organs concerned in the nutrition
of the body should work together or cooperate. To accom-
130
INTRODUCTION TO BIOLOGY
plish this, there are organs which cause the coordination (work-
ing together) of all the organs of the body. These are the nerv-
ous organs of the body — the brain, spinal cord, and nerves.
108. Nervous Organs for Co5rdination. — If we touch a
living frog anywhere on its skin, various muscles of the body
will contract, causing the animal to jump. A similar result
comes from suddenly thrusting a stick before the frog's
eyes, or from making a loud noise. The same thing happens
when the frog sees food, such as a worm ; the muscles con-
tract so that the frog jumps and seizes the worm. These
are examples of coordination
between the external organs
(skin, ears, and eyes) and the
muscles which move the body.
Likewise, whenever any change
takes place in any internal or-
gans, coordinated changes in
other organs, as in the beating
of the heart mentioned above,
are caused by the nervous
organs. The activities of the
nervous organs are due to the
combined working of the nerve-cells (Fig. 49) and their
fibers which connect them with various organs of the body.
A large part of the nervous organs consists of connective
tissues surrounding the nerve-cells and their fibers.
109. Reproduction of Frog. — The processes of digestion,
respiration, circulation, excretion, and nervous activity are
essential for the life of individual vertebrate animals ; but
reproduction is essential only for the procreation of new indi-
viduals to take the place of those which die from accident,
disease, or old age. In Chapter XII the reproduction of vari-
ous organisms is briefly described.
Fig. 49. A nerve-cell and its
fibers, n, nucleus ; ac, main
fiber (axis cylinder). The other
fibers are shorter and with many
branches near the cell.
CHAPTER VII
SOME ANIMAL STUDIES PRELIMINARY TO HUMAN
BIOLOGY: SIMPLE ANIMALS
The frog, the insects, and the plants that have been de-
scribed in the preceding chapters have bodies composed of
many cells, and hence they are called multicellular animals
and plants. There are numerous species of organisms whose
bodies consist of a single cell (unicellular). This lesson
presents some unicellular animals which are not only interest-
ing in themselves, but which help us to understand some of the
human Ufe-processes that will be considered in the next
chapter.
I. THE STRUCTURE AND LIFE OF THE SIMPLEST
ANIMALS
110. One-Celled Animals. — The simplest animals consist
of one cell ; that is, they consist of a small mass of proto-
plasm with a nucleus. Within this one cell must be carried
on all the processes connected with the fundamental life-
activities (feeding, breathing, excreting, reproducing), for
each of which processes an animal like a frog has special
organs with thousands of cells. In order to understand how
an animal with one cell can carry on the same life-activities
fls does an animal with thousands of cells, it is necessary to
study some examples of the simplest animals.
111. Paramecium. — {Dor L) If some chopped hay be placed in
water in a fruit-jar or other convenient vessel, and then to this be
131
132
INTRODUCTION TO BIOLOGY
added some deca3dng sticks, leaves, and other objects taken from
a pond where aquatic plants are growing, there will probably develop
within a few weeks large numbers of transparent animals appearing
to the naked eye as minute whitish specks. With a rubber-bulbed
pipette take a drop of water from the surface and near the edge of
the vessel, and place on an object-slide. Then place a few shreds of
cotton on the slide, and put on the cover-glass.
Place the slide on a piece of black cloth or paper, and notice the
moving white specks. Use a hand-lens. Now examine with a mi-
croscope, using first the low-power objective. Look for rapidly
moving objects having the form shown in Fig. 50. These are
(Oac
^te.gTt
c:vat.
Fig. 50. Paramecium. Ay viewed from below; B, interior of transparent
body; huc.gr, groove for collecting food; c.vac, contractile vacuole; cw,
cuticle; f.vac, food masses; gvl, gullet; mthy mouth; nw, nucleus. {From
Parker.)
SIMPLE ANIMALS 138
specimens of Paramecium, sometimes called the " slipper-animal-
cule." In size, they are about jhs ii^ch long. Several other species
of similar animals are often abundant in the same water.
Study the following points : form of body, movements, response
to stimuli (e.0f., when the animals bump against obstructions).
One end is anterior (i.e., goes forward in locomotion).
Mount and examine another slide with some paramecia which
have been swimming for a half-hour in water containing powdered
carmine, or lamp-black. Notice the action of the lashing cilia
upon the particles of carmine surrounding the animal, and espe-
cially the collection of particles in the groove on one side of the body
and then their entrance into the body through a tube or gullet.
This taking of carmine illustrates how a Paramecium gets food,
chiefly bacteria and other small plants or animals. Notice that
there is no stomach or other cavity into which food is taken, but
simply particles of food surrounded by a film of water are forced
into the protoplasm, which is soft and semi-liquid. These food
masses (called food-vacuoles) move around inside the animal and
digestive juices slowly dissolve the digestible food. The indiges-
tible remains (such as hard parts of some small animals and plants
often eaten, or the carmine particles in above experiment) are ejected
at a soft spot in the cell-membrane.
Using the high-power objective, examine the cilia carefully.
Look for the two clear spots shown in Fig. 50, A ; they are spaces
in the protoplasm filled with water, and when the surrounding
protoplasm contracts the contents are ejected through a little canal
leading out of the body. They are excretory organs, known as con-
tractile vacuoles, and the water they eliminate contains some
nitrogenous excretions. Sometimes they pulsate as regularly as
the heart beats in higher animals.
Stained preparations should be examined for the nucleus. A
small nucleus lies near the main nucleus.
Reproduction by Division. — Some of the paramecia seen
swimming may be constricted near the middle of their bodies,
as if an invisible thread were tied around them. The con-
striction grows deeper, and finally the animal is completely
divided into two new and yomig individuals which swim
mdependently.
134 INTRODUCTION TO BIOLOGY
112. Physiology of Paramecium. — A brief study of the
life-processes of this simple animal will help us when we
study the life of the human body. In some remarkable re-
spects the simplest animals resemble other animals and
humans.
Foods, — Like every other animal, a Paramecium requires
foods that have been made, directly or indirectly, by plants.
These foods are taken in through its mouth and gullet, are
digested by enzymes which are beUeved to be similar to those
which cause digestion in higher animals, and the digested
foods are absorbed by the protoplasm. Finally, the indiges-
tible substances are ejected from the body.
Oxygen, — Like every other animal, it requires oxygen.
It must breathe; but it has no lungs, as have higher ani-
mals. However, its delicate cell-wall allows the absorption
of an abundance of oxygen from the surrounding water.
Paramecia soon die in water from which the oxygen has been
removed.
Oxidation and Excretion, — Like every other animal, a
Paramecium's food is constantly being oxidized to furnish
energy to run the living machine, and this union of oxygen
and foods forms excretions. As in a frog, these are chiefly
water, carbon dioxide, and nitrogenous excretions. The car-
bon dioxide is absorbed by the surrounding water. The
water and the nitrogenous excretions are pumped out by the
regular pulsations of the contractile vacuoles. Thus, with-
out lungs, kidneys, or skin, a Paramecium gets rid of the
same excretions which in the frog and other higher animals
must be eliminated by these organs.
Assimilation, — As in every other animal, some food must
be used continually for making new protoplasm by assimila-
tion. Protoplasm is continually wearing out and some food
must be used for repair. Also, the animals are frequently
SIMPLE ANIMALS 135
dividing, and every new individual is half the usual size and
must make enough new protoplasm to double its size before
it can reproduce. Hence, much food must be used to make
new protoplasm for growth.
No Circulation. — Especially is it noteworthy that, imlike
the frog and other higher animals, a Paramecium carries on
its life-activities without organs for circulation (heart and
blood-vessels). Turn back and review the reasons why a
frog needs these organs (§ 105), and it will be evident that
none of the reasons given applies to a Paramecium, for that
animal is so small that digested food, oxygen, and excretions
do not require transportation to and from distant parts of
the body. It is only a short distance from the surface of the
body to the innermost particles, and so oxygen and carbon
dioxide can diffuse as easily as they go through the wall of a
blood-capillary in a frog. Also, the movement of the inner
layer of protoplasm in a Paramecium helps distribute food
while it is being digested.
Irritability. — A Paramecium has no nerves or other
structures which appear to be substitutes for the nervous
system in a frog ; nevertheless the animal responds to stimuli
caused by chemicals, touch, light, and electricity. It has no
nervous organs, but it has irritability (response to stimuli)
in a simple form.
Motion. — A Paramecium has movement, and yet there
are no visible muscles. Instead, the protoplasm of its entire
body seems to have the power of movement or contractility.
This is especially .developed in the protoplasm of the cilia.
Reproduction. — And finally, a Paramecium has the power
of reproducing, without which its species would soon cease
to exist. Since it consists of a single cell, its only possible
way of producing new individuals is by cell-division, a simple
process of rejyroduction.
136 INTRODUCTION TO BIOLOGY
Summarizing^ we find in this simple animal all the life-
processes which we find in higher animals; namely, move-
ment, digestion, absorption, respiration, assimilation, oxida-
tion, excretion, irritability (simple nervous activity), and
reproduction. All these life-processes are within one cell in
a Paramecium, and require millions of cells in an elephant.
But there is a difference in that the Paramecium cannot per-
form any process as well as can a higher animal. In fact, to
a large extent a Paramecium is doing just what every indi-
vidual cell in any higher animal is continually doing in carry-
ing on its own Ufe-processes.
113. Physiological Division of Labor. — The fact that the
cells in higher animals are specialized to do certain work (e.gf.,
muscle cells to contract or cause movement, stomach cells
to digest, kidney cells to excrete, nerve cells for coordination,
etc.) is known as physiological division of labor. The ad-
vantage of such specialization is shown by an analogy from
human society. Each pioneer in the coimtry regions in
America had to be his own baker, miller, carpenter, black-
smith, cobbler, etc. ; because special workers in these lines
were not near. But such a man never became expert in any
of these lines ; he was " a Jack of all trades, master of none."
Nowadays in civilized communities a sociological division
of labor has led men to become specialists and learn to do one
thing excellently. But there must be a coordination or a
working together. One man may specialize as a carpenter ;
but he must depend upon other men to be his cobbler, grocer,
baker, farmer, and so on through a long list of people who
must do things for the one who confines his work to one
special line. In our great cities we do not often stop to
think of this sociological division of labor which has grown
up in our complex human society ; but if all the grocers were
to close their shops or, as has actually happened, the railroad
SIMPLE ANIMALS
137
en^eers should stop work and leave us without supplies,
then we should realize the complex way in which we depend
upon other workers in special lines of work.
All this from human society illustrates the division of
labor in higher animals. For example, muscle cells have
speciahzed for movement ; but for them other cells must do
the digesting, absorb the oxygen, discharge the excretions,
etc. In short, the cells of every organ depend upon the cells
of all the other organs; and all must work together har-
moniously, because there is mutual interdependence. And
just as a man specializing in one business becomes expert, so
a cell specializing in movement, secreting, or in any other
one necessary function, can do that work better than it is
done in a Paramecium, for that animal has so many things
to do in its one cell that it does none of them as well as they
are done by the specialized cells in higher forms of life.
114. Amoeba. — In many ways the most interesting of
one-celled animals is one belonging to the genus Amoeba,
which lives on the surface of sub-
merged objects in ponds and other
bodies of water.*
The body of an amoeba con-
sists of protoplasm, imbedded in
which are many food particles
that have been eaten and not yet
digested. The outermost proto-
plasm is perfectly transparent
and colorless, while the central
portion is very granular and
resembles ground or frosted glass.
Fig. 61. Amoeba, n, nucleus ;
/, food particles; v>, water
droplets ; cv, contractile vacu-
ole. (After Wilson.)
The microscope and
* To Teachers : For directions concerning collecting and arranging
amoebas for microscopic study, see § 270 in the "Applied Biology" and in
the "Teachers' Manual of Biology." Demonstrate to the students the
chief points mentioned in the text.
188 INTRODUCTION TO BIOLOGY
drawings give .the impression that the animal is a thin sheet
of protoplasm, spread out like a liquid poured on a flat plate ;
but when the animal is disturbed (as by jarring) it rolls up
into a roimded mass.
Movement, — The most noticeable thing about an active
amoeba is that it is continually changing its shape. Close
observation will show that this is due to the fact that the
animal is composed of semi-fluid protoplasm which is con-
stantly flowing. This movement of the protoplasm is not
like the flowing of water down an incline, but is due to con-
tractions originating within the amoeba itself. It is possible
for the animal to crawl up the stem of a water plant, and
this could not be explained by the laws of gravitation.
When the animal is actively moving, long arm-like pro-
jections, called pseudopods (false feet), are formed, and then
granules from the main body continue streaming in the same
direction. In fact, the motion reminds one of the flowing
of lifeless liquids, and we can imitate it by pouring some
thick liquid (molasses, melted gelatin, or mucilage) on a
plate, and then by tilting cause the liquid to flow in various
directions. Notice that when a streamlet starts in one
direction, the liquid all tends to flow in the same direction.
Sometimes a nucleus can be seen in a living amoeba, but
it is very prominent in a stained preparation.
In large specimens it is easy to see the one contractile
vacuole, which acts essentially like those seen in a Parame-
cium, and has the same function of excreting water contain-
ing nitrogenous waste.
Food. — • The taking of food can sometimes be observed
when an amoeba is moving actively. If it comes in contact
with a small animal or plant, two pseudopodia flow out and
gradually surround and inclose the food particle in the pro-
toplasm of the amoeba. A small amount of water surrounds
SIMPLE ANIMALS 139
the food particle, just as in a Paramecium. Sometimes an
amoeba is seen so filled with food particles that the body
substance is opaque. The protoplasm of amoebas secretes
digestive enzymes which dissolve the proteins, etc., in the
animals and plants which are captured. The digested foods
are then absorbed by the surrounding particles of protoplasm,
and when the amoeba moves, the particles being digested are
circulated so that dissolved food is widely distributed. In-
digestible particles (hard parts of small animals and cell-
walls of plants) are from time to time ejected; the proto-
plasm of the amoeba appears to flow away from them as
water on a board might for a time surround and inclose some
grains of sand and then flow
away and leave them behind.
Oxygen, — An amoeba, like a
Paramecium, absorbs oxygen
from the surrounding water.
Excretion is the same as in a
parameciimi ; carbon dioxide is
absorbed by the surrounding
water, and water and nitrogen-
ous excretions are pumped out
by the contractile vacuole.
Reproduction, — Amoebas are
rarely seen dividing (Fig. 52),
but if kept in watch-crystals
they multiply rapidly, making
it evident that they must under- fio. 62. Diagram showing divi-
go division frequently. Under ^^^"^ °^ »^ amoeba, n, nucleus ;
,._ - _- _ r, contractile vacuole.
conditions unfavorable to the
usual activity, amoebas will sometimes become rounded, and
secrete around themselves a hard wall or cyst. They are said
to be encysted. In this condition they are able to live for
140 INTRODUCTION TO BIOLOGY
some time without food and in a dry state. Such a habit
insures their existence in dry seasons when the ponds are
dry. Also, in the dry condition they may be widely scat-
tered by winds.
115. Allies of Amoeba and Paramecium: Protozoa. —
These animals are members of the lowest division of the
animal kingdom, a group of animals which are characterized
by the fact that their bodies consist of one cell. Any one-
celled animal is a protozoan (meaning first animal), and the
entire group of them is named Protozoa. There are numer-
ous species, but most of those known are like the amoeba
and paramecimn in being of interest because a study of them
has helped much towards explaining the life of higher organ-
isms, including man himself. A few protozoans are now
known to be of very great practical interest. Certain species
live as parasites in the blood of men and animals and cause
disease. Examples of diseases so produced are malaria,
and Texas cattle fever. Many protozoans are' important
as scavengers and as the food for larger animals. (See
" Applied Biology,'' § 277.)
For the position of Protozoa in the animal kingdom, see
the table of classification'in § 87 of this book.
II. SIMPLE MANY-CELLED ANIMALS
The one-celled animals represent the simplest animal life,
and the frog belongs to the most complex group (the verte-
brates). In the frog there are many organs, each composed
of thousands of cells. It will now be interesting and useful
to study an animal which has numerous cells, arranged in a
very simple body that is able to perform all the life-processes
found in the frog.
116. Hydra. — Much more differentiated than the one-
celled animals, but exceedingly simple as compared with a
SIMPLE ANIMALS
141
backboned animal, are those of the group to which belong
the jelljrfishes and coral-animals. Most of these live in the
sea, but among the few species which live in fresh water are
the little animals known as hydras, and belonging to the
genus Hydra. They commonly live in ponds and streams,
clinging to aquatic plants, dead leaves, and sticks.
I. (L) Observe hydras in glass jars (aquaria) near windows. In
what part of the aquarium with reference to light and shade are
they most abundant ? Note the long thread-like arms which are
attached at the free end of the an-
imal. Study a hydra which has
been removed from an aquarium
to some water in a watch-glass.
Use a hand-lens. What is the shape
of the body? Does the shape
change? Disturb the animal by
touching it with a needle. Describe
what happens. How may this re-
action be of use to the hydra?
Notice that one end becomes at-
tached to the glass; this is the
hcLse. Find out if the hydra has a
firm hold on the glass dish, e.g,, try
to wash it off with a gentle jet of
water from a pipette. At the free
end b a circle of tentacles. How
many ? These are used for catching
the prey. On the sounmit of a con-
ical elevation in the center of this
circle of tentacles is a small opening,
the mouth, which is usually very
difficult to see: How do the ten-
tacles behave when the hydra is dis-
turbed? Can the tentacles move
independently of each other? Is
this animal bilaterally symmetrical ?
Often young hydras may be seen
Fig. 53. Two specimens of hy-
dra, one contracted and one ex-
panded. The latter has three
buds in stages of development
(6cP' ^' '). <, tentacles; a, captured
water-flea ; hyp, position of
mouth. (From Parker.)
142
INTRODUCTION TO BIOLOGY
attached to the bodies of the larger ones, and these are formed by
outgrowths or huds (Fig. 53).
Make an outline drawing of a living hydra, showing as many
structures mentioned above as possible — (1) fully extended,
(2) contracted.
II. (L) Transfer a hydra, with a little water, by means of a
clean pipette, to an object-slide. Be careful to support the cover-
glass (small bits of broken cover-glasses may be used as supports)
so as not to crush the animal. Examine with low power of the com-
pound microscope. Notice that there are two layers of the body-
wall ; the outer {ectoderm) is colorless, the inner (endoderm) is green
or brown. The green is due to the presence of chlorophyll. In the
outer clear layer, look for knob-like
swellings, especially on the tentacles.
These swellings contain the stinging
or nettle threads, which are organs of
defense, and also used for spearing and
paralyzing water-fleas and other small
water animals.
III. (DorL) Microscopic Sections.
— Examine with a compound micro-
scope a permanent slide with longi-
tudinal sections of hydra and ob-
serve : —
A hydra is really a hollow cylinder,
the interior of which is the digestive
cavity (Fig. 54).
The tentacles are hollow outgrowths
of the digestive cavity.
The mouth is the only opening from
this cavity to the outside.
The body-wall is composed of layers
of cells on both sides of a deeply-
stained line in the preparation. The
outer layer is the ectoderm: this is
a living hydra. The deeply-stained
line (middle layer) is not made up of cells, but consists of a gelat-
inous substance. The inner layer of large cells is the endoderm.
Look for these three layers in a section of a tentacle. In the
ectoderm of the tentacle look for the stinging cells or nettle-ceUs
Fig. 54. Longitudinal section
of a hydra, t, tentacle; m,
mouth; e, ectoderm or outer
layer of cells; n, endoderm
or inner layer; d, digestive
cavity; b, buds; s, spermary;
o, ovary. (From Parker.)
the colorless layer seen in
SIMPLE ANIMALS 143
(Pig. 55). Each of these contains a sac filled with fluid and sur-
roimding a coiled stinging thread. If a small animal happens to
touch a hair-like projection (trigger) on the outer
edge of one of these cells, the stinging thread is
suddenly ejected and spears the victim. It
appears that some paralyzing fluid is injected,
for the animal caught seems benumbed. Finally,
the tentacle bends over towards the mouth and
the captured food is taken into the digestive cav-
ity. The nettle-cells may also be used for defense d
against animals too large for the food of a F'o- 56. a, sting-
hvdra ^^^ ^^ ^*^
^^ * thrc&d inclosed *
If there is a bud present on the section, notice 5 thread di&^
the relation of its digestive cavity to that of the charged. Those
parent. o^ *^® tentacles
Study a transverse section of a hydra and . ? ^ f^ ^
compare with the structure in the longitudinal Hatachek.)
section.
117. Physiology of Hydra. — The life-activities of a
hydra are more specialized than those of one-celled animals,
but they are exceedingly simple as compared with those of
an animal like the frog.
Digestion, — Small animals caught by the tentacles and
forced into the digestive cavity through the mouth are
softened and disintegrated by the action of the digestive
juice secreted by the cells of the endoderm which lines the
cavity. The digestible particles set free by the disintegra-
tive action of the digestive juice are taken in by endod6rm
cells, the free ends of which are amceba-Iike and able to take
in food in the way that an amoeba does. Particles thus
taken into the cells are completely digested (dissolved).
This is intra-cellular digestion (ie., within cells), and is the
same as the only possible digestion in Protozoa and Porifera.
But in the hydra there is a decided advance over the lower
animals in that the cells able to digest intra-cellularly are
144 INTRODUCTION TO BIOLOGY
grouped so as to form the lining of a cavity (digestive cavity)
into which they pour secretions that cause digestion outside
of the cells. This digestion in a cavity, as in the stomach
of all higher animals, is extra-cellular digestion, i.e., occurs
outside of cells. The digestive cells in a hydra, then, in part
resemble the one-celled animals and the digestive cells of
sponge-animals ; and in part the lining cells of the stomach
of higher animals.
Absorption of Digested Food. — The cells (endoderm)
which line the digestive cavity of a hydra can either get food
by taking in particles and digesting them inside of cells, or
by absorbing foods digested in the cavity. The cells of the
outer layer (ectoderm) must get all their food by absorption
or osmosis from the endoderm cells. This osmosis of food
from cell to cell is accomplished as easily as water soaks
through several layers of paper.
Use of food in cells of the hydra is the same as in all other
animal cells. Some food is used to make new protoplasm
for repair and growth, and some serves as a source of energy.
Oxygen is absorbed by all cells in contact with water.
This includes the majority of the cells, for even the digestive
cavity contains water taken in with the food. But probably
the larger part of the oxygen needed is absorbed from the
water by ectoderm cells.
No Circulatory Organs. — Although a hydra is multicellular,
it is able to live without circulating blood and lymph for the
reason that its simple cylindrical body allows digested food
and oxygen to reach all cells by osmosis, and all excretions
are eliminated by the same process. Compare this with the
distribution of digested food, oxygen, and excretions in a frog.
Excretion is accomplished by osmosis of waste matters
from the cells to the surrounding water. Probably the ecto-
derm cells do the larger share of this work.
SIMPLE ANIMALS 146
Movement is caused partly by contraction of cells and
partly by that of muscle-processes which are extensions of
some cells in both inner and outer layers.
Irritability is well developed in the hydra. It is in part a
function of all the constituent cells ; but some cells believed
to be simple nerve-cells are near the nettle-cells, which must
be extremely sensitive in order to discharge the nettle-
threads when stimulated by other small animals.
Reproduction of hydras is sometimes by budding as shown
in Fig. 54, and at other times from fertilized eggs. This
process will be described in § 332.
118. Division of Labor in Hydra. — All cells must use
food, but only the endoderm cells are able to digest food.
Hence, ectoderm cells must depend upon the endoderm cells
for their food. On the other hand, the ectoderm cells form
the external protective covering of the animal, furnish the
cells which form the reproductive organs, catch the food,
receive impressions from the environment, and probably are
the chief causes of the movements of the animal. The ten-
tacles are specialized for collecting food, and their nettle-
cells still more highly adapted to a special purpose. The
presence of simple nerve-cells suggests the beginning of dif-
ferentiation of nervous organs.
119. Classification of Hydra. — The simplest animals are
the one-celled Protozoa (§§ 110-115), next highest in com-
plexity are the sponge-animals (Porifera), and then next
are the coelenterates (Ccelenterata), examples of which are
hydras, sea-anemones, jellyfishes (medusae), ctenophores,
and coral-animals. For descriptions and pictures of any of
these, consult the indexes of any textbooks of zoology or
the author's " Applied Biology." The flat-worms and round
worms are the groups of animals just above the coelenterates
in complexity of structure. The reader should bear in mind
146 INTRODUCTION TO BIOLOGY
that " higher " as applied in comparing animals and plants
means more complex in structure, and vice versa, " lower "
means simpler in structure. We classify organisms entirely
according to structure, grouping together those that are
most similar.
See the position of the coelenterates in the table of classi-
fication in § 87.
CHAPTER VIII
HUMAN STRUCTURE AND LIFE-ACTIVITIES
120. Human Biology. — Biology is the science of living
things, and human biology may be defined as the study of
man considered as a living thing and interpreted in the light
of studies of other living things.
The justification for including study of man as part of
biology is found in the fact that the human body in its
structure and functions is remarkably like that of animals,
the higher forms in particular. In the body of man are the
same organs as in the animals known as beasts or mammals ;
and the organs of man and the beasts are closely alike even
in microscopic details. This similarity also appears when
comparing man with still lower animals. In short, when
biologists consider the close similarity of structure and func-
tion in man and various types of animals, they see no escape
from the conclusion that man's relation to the animal king-
dom is as stated in the next paragraph.
121. Classification of Man. — (1) As suggested above,
man belongs to the animal kingdom, because his body is
built on the plan of structure found in many animals.
(2) Man is a backboned or vertebrate animal, because he
possesses a backbone or vertebral column. (3) Man belongs
to the class of mammals or MammaKa, because he has the
three characteristics of this group, — hair, diaphragm, and
milk or mammary glands. (4) Mail belongs to the order of
the Primates, because his body is in numerous respects more
147
148 INTRODUCTION TO BIOLOGY
similar to apes than to other animals. (5) Man belongs to
the human family, the genus Homo, and there is now only
one species, sapiens (a word meaning ''wise," and referring
to the fact that man's intellectual development is charac-
teristic and markedly distinguishes the hmnan species from
all other animals). In fact, it is in the highly developed
functioning of the nervous system alone that man stands
distinctly differentiated from the highest apes and other
animals. This, however, is the proper field of the science
of psychology (the study of mind or mental phenomena),
which should be taken up in college or in private reading
after graduation from high school. So far as biology is
directly concerned, it has to take into consideration simply
the demonstrated fact that man and other animals are re-
markably similar in structure and functions. This similarity
is fortunate, for it makes possible the application to human
biology of many facts which were first learned by the study
of various animals. (6) Finally, the existing human species
has five varieties or races, — Caucasian, American Indian,
Mongolian, Malay, Ethiopian, — each with certain pecu-
liarities.
(L) Look up these races in any advanced textbook of geography,
or in an unabridged dictionary, and report briefly concerning their
characteristics and geographical distribution.
Study of the customs, character, history, and institutions
of races of men is the science of ethnology. The science deal-
ing with the laws of human society is sociology. General
study of man, combining facts of ethnology, biology, psy-
chology, and sociology, is anthropology. The following
brief definitions in parentheses will help the memory : psy-
chology (science of mind) ; sociology (science of society) ;
ethnology (science of human races) ; anthropology (science of
man). Each of these lines of study of human life is now so
HUMAN STRUCTURE AND LIFE-ACTIVITIES 149
highly developed that special books are necessary ; but they
all are founded on biology to such an ejctent that a knowledge
of that science is important for the reader of any of the special
sciences dealing with man.
122. General Plan of Human Body. — like the frog,
already studied, the human body consists of head, trunk,
and Umbs. The trunk is composed of the chest or thorax,
and the belly or abdomen. The arms are the upper or an-
terior limbs; the legs are the lower or posterior limbs.
The thigh of a leg corresponds in structure to the upper
arm, the shank to the forearm, the ankle to the wrist, the
toes to the fingers. Externally the body is bilaterally sym-
metrical.
123. Skeleton. — Like the frog and all the other back-
boned animals, the human body is supported by an internal
framework or skeleton composed of bone and cartilage, the
latter chiefly at the ends of bones. There are more than 200
bones in the human skeleton; 33 vertebrae or segments of
the spinal column, about 25 bones in the skull of an adult,
24 ribs, 30 bones in each arm and leg, and the bones in the
shoulder-girdle and pelvis. The number of bones varies
with the age ; for example, the skull bones are more numer-
ous in yoimg children, but they grow or fuse together as the
individual becomes older. Each half of the pelvis is com-
posed of three bones which have fused together. Nine bones
(vertebrae) at the posterior end of the spinal column are
fused together in connection with the pelvis, leaving 24
separate vertebrae.
(L) The best way to study the human skeleton is to compare a
mounted skeleton with labeled drawings (FHg. 56). At the same
time the pupil should locate the position of the larger bones in his
own body. If a mounted skeleton is not owned by the school, the
pupil should locate as nearly as possible the bones in his own body,
160
INTRODUCTION TO BIOLOGY
Fig. 56. a, skull; h, neck or cervical vertebrae; c, clavicle; d, sternum; e,
scapula; /, humerus; g, radius; h, ulna; i, carpals (wrist); A;, hand; I, ribs;
m, sternal cartilage; w, vertebrsB; o, coccyx; p, pelvis; r, femur; «, patella
or knee-bone; t, tibia; w, fibula; v, heel; w, ankle; 2, toes.
HUMAN 8TBUCTUBE AND LIFE-ACTIVITIES 161
using labeled pictures as a guide. The names of the large bones
are so frequently referred to that it is desirable to memorize them.
Pupils who are studjdng drawing will find various parts of the skele-
ton good objects for sketching ; but as a rule this work is not possible
in the limited time available for the biology class-work.
The most important parts to notice while examining a skeleton
are : (1) The backbone or vertebral column, to which all other parts
of the skeleton are attached. It is the central supporting axis of
the body. (2) The bones of the two pairs of limbs, comparing the
anterior with the posterior pairs. (3) The ribs and the bones which
connect the arms with the backbone. (4) The pelvis, which con-
nects the legs with the backbone. (5) The larger bones of the
skull.
124. Body-Wall and Body-Cavity. — As in the frog, the
outer, fleshy wall which incloses the internal organs is the
body-wall; the internal cavity is the body-cavity. In the
frog there is one cavity in which lie the heart, lungs, liver,
stomach, intestine, kidneys, and reproductive organs. In
the human body the diaphragm forms a partition across the
body-cavity, dividing it into the anterior (upper) cavity
containing the heart and limgs and known as the thoracic
cavity (chest-cavity), and the posterior (lower) abdominal
cavity, which, contains all the internal organs except the
heart and Imigs. The thoracic cavity is inclosed by the ribs,
while the abdominal cavity is bounded by the muscular walls
of the abdomen.
StriLcture of the Body- Wall, — (D) This is essentially the same in
man and other mammals, and so we may study any of the ani-
mals found in meat-markets. A slice of bacon will serve our pur-
pose. On the one edge of the slice is the skin or ** rind." This,
of course, was the skin of the pig. The streaks of lean meat are
muscles of the body-wall. Fat has been deposited in the connective
tissue between the muscles, and also between the muscles and the
skin. The thickness of the body-wall depends upon the amoimt
of fat ; hence very lean bacon is relatively thin. Notice that the skin
152 INTRODUCTION TO BIOLOGY
is fastened firmly to the muscles and fat ; this is due to fibers of con-
nective tissue. In some parts of the body of mammals the skin is
not fastened down so closely as in bacon ; e,g., note the loose skin
on the back of your own hand, or on the backs of young puppies.
126. Relative Positions of Internal Organs in Man. —
Study diagrams (Fig. 57), charts, and a manikin, if avail-
able ; and note the following positions of the largest organs,
which are in the same relative position as in the frog. (1) The
body-cavity is ventral to the backbone. (2) The alimentary
canal extends through the body-cavity from anterior to pos-
terior. (3) The heart is ventral to the alimentary canal
(esophagus part) . (4) The liver lies ventral to the alimentary
canal (stomach and intestine part). (5) The kidneys lie
dorsal to the alimentary canal and in the posterior part of
the body-cavity (abdominal part). (6) The brain and spinal
cord lie in the cavities formed by the bony skull and the
backbone.
126. Life-Activities of the Human Body. — The charac-
teristics of living things already studied in connection with
animals and plants apply to the human body, for it is a
Uving mechanism which performs the functions necessary
for life.
The life-activities are located in the cells; and these, as
in the case of the frog, are in the tissues (epitheUal, con-
nective, muscular, bony or osseous, cartilaginous, and
nervous).
(D) If preparations of human tissues are available, they should
be exhibited and compared with Figs. 39-44. Epithelial cells should
be scraped gently from the lining of the mouth and mounted under
a cover-glass on an object-slide. Examine first unstained, and then
stain by drawing (with absorbent paper) some iodine-eosin or other
dye under the cover-glass. Note that these cells overlap ; it is be-
cause there are many layers of flattened cells in the epithelium of the
mouth. In fact the Uning of the mouth has the same structure
HUMAN STRUCTURE AND LIFE-ACTIVITIES 153
Fio. 67. A, aorta (main artery); B, bladder; Bl, bronchial tubes; D, dia-
phragm; G, gall-bladder; H, heart; L, lungs; La, pulmonary artery; LI, large
intestine; Lv, liver; P, pericardium; R, ribs; Ra, right auricle; S, stomach;
5/, small intestine; V, main veins. (From Schilling.)
154 INTRODUCTION TO BIOLOOY
as the skin on the lips. A piece of homy outer layer of skin may be
scraped from a calloused spot on the palm of the hand, and after
some soaking in a drop of caustic soda solution, the dried scale-like
cells will swell.
Probably the two activities of the human body which
most attract our attention are eating and breathing. More-
over, these are the basis of all the other processes occurring
in the human body. For these reasons we shall specially
consider the taking of foods and oxygen into the human
body, and later we shall trace the changes of foods and
oxygen in the body. This will lead us on to consider all the
essential life-activities.
FOODS
127. What are Foods ? — A convenient definition for our
present purposes is that foods are solid or liquid substances
which when taken into the alimentary canal are useful in the
life-processes of our bodies. In most cases substances able
to serve as our foods must be capable of being digested and
absorbed as materials for energy, .repair, and growth ; but
a certain amount of indigestible plant tissue is useftd in the
human alimentary canal.
Sources and Kinds of Human Foods. — (L) Write in your note-
book a list of some common foods, arranging in three columns those
of animal, plant, and mineral origin.
128. Nutrients. — We might consult cook-books and make
a very long list of the names of prepared foods which are
served on our tables ; but these are made by combinations
of such common ingredients as meats, vegetables, milk,
butter, lard, sugar, flour, starch, chocolate, salt, etc.
Chemists have shown that these common things which are
used in every kitchen in combining our foods are composed
HUMAN STRUCTURE AND LIFE-ACTIVITIES 155
of certain chemical compounds known as sugar, starch, fat
(oil), proteins, and minerals. These compounds, from which
all the combinations of human foods are made, are known in
physiology as nutrients.
In order to prove that various foods are made up of these
few nutrients, we need to know some method of identifying
each. Fortunately, chemists have discovered some simple
tests which are easily applied, as directed below.
129. Chemical Tests for Nutrients. — Certain chemicals produce
characteristic reactions on nutrients and hence may be used to
detect their presence in mixtures of foods.
Starch Test. — (D or L) Kepeat the iodine test with diluted
Btaroh paste (§64).
Sugar' Test. — (D or L) Boil a few grapes, raisins, or prunes in
water in a test-tube for two minutes, and this will extract some sugar.
Into another test-tube put some Fehling*s reagent,* the ingredients
of which are usually kept in separate bottles and mixed when needed.
Heat the Fehling's reagent, and then with a pipette slowly drop
into it some of the sugar solution obtained from the dry fruits ; or
drop some of the reagent into the sugar solution. A red colored
precipitate indicates the presence of one kind of sugar. One such
experiment could not prove that when Fehling's reagent causes a
red color it means that sugar is present, for we have not yet tried this
solution on starch, fat, and other things. However, chemists have
tried the Fehling's reagent on all the substances commonly found in
animals and plants, and it has been demonstrated that only certain
kinds of sugars produce the red color. We have not time to repeat
such investigations, and so we must accept the chemists' statement
that Fehling's reagent is a test for a sugar found in grapes and known
as grape-sugar or dextrose. The sugar in corn-sirup is popularly
called glucose. Its chemical formula is CaHizOe.
Tests for Fat, — ( D or L) Put a drop of olive oil on a sheet of white
paper, and note that a grease-spot is produc'ed. Dissolve some beef-
* Fehling's reagent, a test for certain kinds of sugar, may be purchased
from dealers in chemicals or at ordinary drug-stores. It may be made ac-
cording to the formula in the "Teachers' Manual of Biology," which ac-
companies this book.
156 INTRODUCTION TO BIOLOGY
suet in a small quantity of benzine or ether (keep such volatile
liquids as these far away from a flame), put a drop of the solution on
paper, and notice the spot left after evaporation of the benzine.
Or lay a piece of suet on a paper and heat slowly. This " grease-
spot " test is a simple way of finding whether abundant fats or oils
are present in foods.
Te8t8 for Protein. — (D or L) Mix a small quantity of white-of-
egg in water in a test-tube, shake well, add some strong nitric acid,
boil until the solution turns yellow, cool, and then add drops of am-
monia until an orange color appears. Or instead of the acid, add to
the egg-albumen in water some drops of Millon's reagent (mercury
dissolved in nitric acid, obtainable from chemists), heat slowly, and
red color will appear, especially after cooling.
Tests for Water and Minerals. — The loss of weight by drying
foods is chiefly due to evaporation of water. The ashes left after
burning foods represent the mineral contents. By complicated
processes chemists can analyze the ashes and determine the kind and
proportion of elements present.
130. Sugars and Starches : Carbohydrates. — All sugars
and starches are grouped together under the name of carbo-
hydrates. They contain but three elements : carbon, hydro-
gen, and oxygen. We have already learned that starch is
formed in plant cells which have chlorophyll, and that
starch is readily digested by enzymes into sugar, or sugar
turned back again to starch for storage. The carbohydrates
which are used as human foods are chiefly cane-sugar or
sucrose and milk-sugar or lactose (both with the formula
C12H22O11) ; grape-sugar and fruit-sugar (both with the
formula C6H12O6) ; and starch and dextrin (whose formulas
are some multiple of CcHioOb).
As we have seen, grape-sugar is found in raisins and other
fruits. Under the names of glucose and corn-sirup it is com-
mon in the markets, and is made by treating the starch of
com grains with strong sulphuric acid. Lactose or milk-
sugar is sold in all drug-stores for use in preparing foods for
HUMAN STRUCTURE AND LIFE-ACTIVITIES 157
infants and invalids. Malt- or barleynsugar (maltose) is also
sold, especially for flavoring candies, etc. The common
" granulated " sugars in our markets are sucrose from the
juices of sugar-cane and sugar-beet. The yellow and brown
sugars are the crude ones obtained by evaporating the
juice pressed from cane and beet ; and by refining processes,
these dark-colored sugars are made into the "soft white "
and " granulated " sugars. Maple sugar, from the sap of
maple trees, is chemically the same as cane-sugar, but flavored
with peculiar substances foimd in the maple sap.
131. Fats. — This includes all kinds of fats and oils from
animals and plants. Conmion examples are butter, lard,
beef-suet or tallow, olive oil, cotton-seed oil, fat of meats,
oil of nuts. Like the sugars and starches (carbohydrates),
fats contain only three elements: carbon, hydrogen, and
oxygen ; but they have much carbon. The sugars, starches,
and fats taken together are often called " non-nitrogenous
foods," because they have no nitrogen.
132. Proteins. — Formerly spelled " proteids." Also
called albumens. White-of-egg (*' egg-albumen ^^), lean meat,
and milk curds have abundant proteins. They contain
carbon, hydrogen, nitrogen, oxygen, and sulphur. Also,
some have phosphorus and iron. All proteins differ from
carbohydrates and fats in having nitrogen and sulphur. As
already stated, only plants can make proteins, and animals
must get them directly or indirectly from plants.
Gelatin is the most common example of certain food sub-
stances which have the composition of proteins, but which
cannot take their place as food. Dogs have been found to
live well if fed pure protein from lean meat; but with
gelatin alone they so^n begin to lose weight and to show
other evidences of starvation. Hence, gelatin must be used
with other protein foods.
168 INTRODUCTION TO BIOLOGY
133. Inorganic or Mineral Foods, and Water. — The in-
organic foods are the only ones not formed by animals or
plants. As we have seen, water plays an important part in
all living matter. In the himian body it is especially im-
portant in dissolving foods during digestion, and also as the
circulating medium in the blood- and lymph-systems.
Conmion salt (sodium chloride, NaCl) is only one of a
number of mineral salts necessary in the human body. A
compound containing iron gives the red color to blood;
lime (calcium) is necessary in the bones ; and less noticeable
quantities of other elements (P, K, S, Mg) are needed in the
human body. Most animal and vegetable foods which we
commonly use contain these necessary elements, and so we
do not have to give any special attention to obtaining them.
Conmion salt is the only mineral food which is regularly
added to our diet in addition to what is naturally in our
organic foods; and it is probably true that we use this
greatly in excess of what the body actually requires.
134. Foods are Combinations of the Nutrients. — From
the foregoing experiments, it is evident that one or more
of the nutrients is present in each of the conmion foods.
Chemists have proved that the useful constituents of all our
foods are carbohydrates, fats, proteins, mineral salts, and
water. The experiments below show that some fodds have
a preponderance of certain nutrients; e.g.j white-of-egg is
chiefly protein, potato is chiefly starch, comnsirup is almost
pure sugar, butter and beef-suet are largely fat. It is evi-
dent that if all the nutrients are needed in human diet, they
can best be obtained by a mixture of foods ; that is, a meal
composed of lean meat, potato, bread, butter, and some
form of sugar might be arranged to supply equal amounts of
protein, fat, sugar, and starch. We shall see later that
equal amoimts of these nutrients are not needed ; but that a
HUMAN STRUCTURE AND LIFE-ACTIVITIES 159
mixture of foods is necessary in order to give the proper
amount of each of the nutrients of which our common
articles of food are composed.
Testing Foods for the Nutrients, — (D or L) Apply the tests
for starch, proteins, and grape-sugar to oatmeal, flour, white-of-egg,
©gg-yolk, potato, onion, rice, beans, peas, lean meat, apple, honey,
com-sirup, pears, corn-meal, and other common articles of food.
Place each food to be tested in some water in a test-tube, boil for a
few minutes, and then pour in the testing reagent to be used. Make
a table in your note-book and record the results of the tests by
making a mark in the proper columns for the nutrient found to be
present.
Name of Food Tssted
Contains
Sugar
Contains
Starch
Contains
Protein
Contains
Fat
STRUCTURE OF HUMAN DIGESTIVE ORGANS
In order to understand many points concerning the work
of the organs which deal with foods, we must first get a clear
idea of the general structure of the alimentary canal and of
its attacked organs which secrete digestive fluids (liver,
pancreas, salivary glands). Therefore we must for a time
turn aside from considerations of function and study the
structures of these organs.
136. The Mouth-Cavity. — (L) Turn your back to a window or
a lamp, and with a hand-mirror reflect the light into your open
mouth. Notice the hard palate forming the roof of the mouth-
cavity. At the back of the mouth-cavity is the soft palate, which
separates the mouth-cavity from the post-nasal cavity; and this
cavity in turn communicates with the cavities of the nose. Take
160 INTRODUCTION TO BIOLOGY
** short breaths " and notice the effect upon the soft palate. Apply
your tongue to the roof of the mouth and slowly move it backward
and forward until you feel the shape, position, and texture of the
hard and soft palates. Press down upon the tongue with a clean
(sterile) glass rod, or the handle of a spoon, and examine the small
prolongation of the soft palate which touches the tongue when that
is not depressed. This is the uvtUa,
136. The Teeth. — (L) Examine your teeth, again using the
hand-mirror, taking the following description as a guide : Beginning
at the middle -line at the front of each jaw, there are in order the
following kinds of teeth in half of either the upper or the lower jaw :
First, two chisel-shape cutting teeth (iTicisarSj meaning to cut into).
Next, a tooth with a more i)ointed edge, which corresponds to the
great fangs of dogs and cats and other animals which must tear their
prey ; hence the name canine or dog-teeth. The tusks of boars and
walruses are enormously enlarged canine teeth. Elephants' tusks
are upper incisors. Next back of the canine tooth on each side there
are in the first or ** milk-set ** of teeth two grinding teeth {molars).
This makes a total of twenty teeth in the first or milk-set, which are
deciduous. In adults there are in each half of a jaw two teeth called
bicuspids (meaning two cusps or points) in place of the two molars
of childhood; and back of these are three molars, often called
" wisdom teeth." There are, therefore, twelve molars in adults in
addition to teeth in the places occupied by the twenty teeth of the
first or deciduous set, making a total of thirty-two for the adult.
The incisor teeth begin to appear in children at six or
eight months of age, and the full milk-set is present after
eighteen to twenty-four months. The loss or shedding of
these, caused by growth of new teeth below, occurs at vari-
ous times between seven and twelve years of age. The per-
manent teeth begin with the incisors at seven or eight years
and are completed with the appearance of the molars or
wisdom teeth at between sixteen and twenty years of age.
The growth of the teeth through the fleshy tissue (gums) is
often called " cutting teeth."
The structure may be studied by breaking open an ex-
HUMAN STBUCTUEE AND LIFE-ACTIVITIES 161
tracted tooth, or better by studying a thin section prepared
for microscopic use. There is a central cavity which, during
the Ufe of the tooth, is filled with a soft mass composed of
connective tissue, blood-vessels, and nerves. This is the
so-called pulp. The pulp-cavity extends down into each of
the roots of the tooth, and at the tip of a root is a small
opening through which nerves and blood-vessels enter. The
hard outer part of a tooth consists of the enamel, and the
main bulk is dentine, a kind of ivory. The roots of the
tooth, which are buried in holes or sockets of the jaw-bone,
are covered with a thin layer of bony substance, called
cement.
137. The Tongue. — Examine with a hand-mirror. The
elevations on the upper surface are papiUce, and nerve-
fibers connect these with the brain. Their function is that
of taste and touch. The tongue is chiefly muscular tissue,
the muscle-fibers extending longitudinally, transversely, and
perpendicularly. Can you think of any relation between
such arrangements of the fibers and the possible movements
of the tongue?
138. Salivary Glands. — The epithelium which lines the
mouth is coated with a limited amount of fluid known as
mucus, hence it is called a mucous membrane. But most of
the fluid in the mouth is saliva from three pairs of salivary
glands. On either side of the head a gland lies beneath and
in front of the ear. These are the parotid (meaning beside
. the ear glands), and a disease affecting them is called paro-
titis * or mumps. A duct from each parotid gland opens into
* Notice that the ending itis added to the name of the organ (parotid)
means inflammation or disease of the organ. Likewise, there are in common
use such words as appendicitis (inflammation of the appendix of the intes-
tines), gastritis (of the stomach), laryngitis (of the larynx), tonsillitis (of the
tonsils), and many other diseases designated by adding itis to the name of
the organ involved.
M
162
INTRODUCTION TO BIOLOGY
the mouth-cavity on a little elevation on the inside of each
cheek opposite the second upper molars. The elevation can
be seen by using a mirror and
holding the cheek away from the
jaw. The two other pairs of
salivary glands are placed among
the muscles beneath the tongue,
and their ducts open into the
mouth-cavity beneath the tip of
the tongue. The secretions of
the salivary glands will be de-
scribed later when we take up
their work in. the digestion of
foods.
139. The Pharynx or Throat-
Cavity. — Again using the hand-
mirror, notice that at the back
part of the mouth-cavity mus-
cular folds extend downward
from the soft palate and bound
laterally the passage from the
mouth-cavity to the throat-cav-
ity. Between these folds on
each side is a round body, tonsil.
These are the organs which often
become enlarged during a " cold"
in the throat (tonsillitis). They
are not known to have any essen-
tial function, and surgeons fre-
quently remove them when they
become permanently enlarged.
The pharynx opens above into the post-nasal cavity be-
hind the soft palate ; and below into the esophagus and the
Fig. 58. Diagrams showing re-
lations of respiratory and ali-
mentary passages in a fish (/),
amphibian (//), reptile or bird
(///), and man {IV). Arrow
A marks passage to respiratory
organs ; B, to alimentary or-
gans ; N, nostrils ; K^ gill-slits;
D, alimentary canal; L, lungs;
O, esophagus; T, trachea; F,
backbone. {After Wiedersheim.)
HUMAN STRUCTURE AND LIFE-ACTIVITIES 163
trachea or windpipe. Refer back to your study of the
throat of the frog. Also, there open into the pharynx the
Eustachian tubes from the ears. The pharynx is the pas-
sage between the nose-cavities and the trachea, and be-
tween the mouth-cavity and the esophagus, thus providing
for movement of air from the nose to the lungs and of food
from the mouth to the esophagus (Fig. 58).
140. The Esophagus or Gullet is a tube extending from
the pharynx to the stomach. In the human body, and in all
mammals, it extends through the. diaphragm, the membrane
which divides the body-cavity into thoracic and abdominal
portions. In the neck the esophagus is behind (dorsal to)
the trachea.
141. The Stomach is a muscular and greatly expanded
portion of the alimentary canal or. digestive tube between
the esophagus and the intestine. It lies on the left side and
in contact with the liver. Imagine a membrane (diaphragm)
stretched across the frog's body-cavity so as to separate the
heart and lungs from the stomach and liver. The human
diaphragm lies in the same position ; that is, the heart and
lungs lie above it (anterior) and the stomach and liver lie
just beneath (posterior). The end of the stomach connected
with the intestine is provided with a muscular ring (pylorus)
which, by opening and closing, is able to control the passage
of food from the stomach to the intestine. It should be re-
membered that the size of a stomach, like that of a rubber
bag, depends upon the amount of distension. When empty,
it is contracted so that it has practically no cavity, and
when much distended by food, it may hold about a half-
gallon.
142. The Small Intestine. — The part of the intestine
connected with the stomach is smaller in diameter than the
extreme posterior part, and hence is called the small intestine.
164
INTRODUCTION TO BIOLOGY
It is very much coiled, as shown in the center of Fig. 59,
and is about twenty feet long.
143. The Large Intestine is about five feet long, which is
one-fourth the length of the small intestine ; but the name
refers to the larger diameter.
As shown in Fig. 59, the large
intestine extends upward, from
the point of imion with the
small intestine, then trans-
versely, then downward. This
largest portion is often called
the colon. The terminal or pos-
terior portion of the large in-
testine, conmionly called the
rectum, is smaller in diameter
than the colon part; and its
external opening is the anus.
Between the colon and the
rectum there is an S-shaped
loop (sigmoid flexure) ; and near
the jimction of the large and
small intestines is the vermi-
form appendix (or simply ap-
pendix), a tube from two to four
inches long and one-fourth of
an inch in diameter. Its in-
flammation, due to bacteria, is
appendicitis, for which the usual
cure is surgical removal of the
organ. It is not necessary in
man ; but in the rabbit and many lower herbivorous animals
it is large and important in digestion.
144. Liver and Pancreas. — These two organs should be
Fig. 69. Human alimentary or-
gans. 2, lung; e, esophagus;
d, diaphragm; 8, stomach; p,
pancreas; I, liver; «t, small in-
testine; », valve between small
and large intestine; ap, appendix;
c, colon of large intestine; r, rec-
tum; a, anus. (From Wieder-
aheim.)
HUMAN STRUCTURE AND LIFE-ACTIVITIES 166
named in the list of digestive organs, for they secrete fluids
which are poured into the intestine by ducts. The position
of the Hver has been described in connection with the stomach.
The pancreas, which in the case of some animals used as hu-
man food is called *' stomach-sweetbread,^'* lies near the
junction of the stomach and the small intestine. Its main
duct (pancreatic duct) joins the bile-duct from the liver, and
the fluids secreted by the two organs are poured into the
small intestine a short distance from the stomach.
146. Glands. — The liver, pancreas, saUvary glands, and
gastric glands have been mentioned as producers of special
secretions useful in digestion. Microscopic study shows that
a gland is composed of a layer of epithelial cells which rest
on a bed of connective tissue. In this latter tissue are
blood-vessels which supply food and oxygen to the cells of
the glands ; and from the materials thus obtained the cells
manufacture their secretions, which they discharge at the
free end of the cells {i.e., opposite the end where they
absorb from the blood). (See Fig. 60.)
(D) Exhibit and describe sections of frog's stomach to show
structure of simple gland. Pancreas sections show numerous tubes
cut across at various angles. The cells of these tubes secrete pan-
creatic fluid.
THE WORK OF THE DIGESTIVE ORGANS
We have briefly considered the structure of the digestive
organs, and it is now our problem to study the work of these
organs, especially with reference to the preparation for ab-
sorption of the different kinds of food; i.e., the nutrients.
As in the frog, this digestion of foods is caused by secretions ;
♦ The neck- or throat-sweetbread sold in meat-markets is from the
thjrmus, an organ found only in young animals, such as calves and lambs,
and l3ring in the anterior part of the chest-cavity close to the neck.
166 INTRODUCTION TO BIOLOGY
and in the human body these are saliva from the salivary
glands, gastric juice from the glands in the wall of the stomach,
intestinal juice from the glands in the wall of the intestine,
pancreatic juice from the pancreas, and hile from the liver.
146. Mechanical Processes in Digestion. — Various move-
ments, due to the action of muscles in the digestive organs,
play two important parts : (1) in taking food and in mov-
ing it along through the digestive tube, and (2) in separat-
ing more or less solid food into small particles upon which
the digestive fluids can act rapidly. The muscular action of
the lips and jaws in taking and chewing (masticating) food
is so easily observed that no description is necessary. The
mastication process is important in that it results in mixing
saliva with the food (see § 205, on hygiene of eating).
After mastication come the swallowing movements, which
are produced by the muscles of the pharynx and esophagus.
These movements are in the beginning voluntary (i.e., under
control of the will) ; but it is well known that when food has
started down the esophagus, the muscular walls of that
organ contract (narrow the diameter) involuntarily, and
force the food into the stomach. The fact that food is
forced down and does not run because of gravity explains
how an acrobat can perform the feat of drinking water
while standing on his head. Horses and cows are examples
of animals which regularly drink and eat with the mouth-
cavity much lower than the stomach.
(D) Take a piece of soft rubber tubing about two feet long, sus-
pend vertically, plug the lower end with a cork, and fill with water.
Now, begin at the plugged end and squeeze the tubing between a
thumb and finger and then move your hand upward so as to contract
the tube in succession. It is evident why the water goes up-hill in
the tube. Similarly, the muscles which are in the walls of the
esophagus contract in succession from the pharynx toward the
stomach and force the food along the esophagus.
HUMAN STRUCTURE AND LIFE-ACTIVITIES 167
The muscle-fibers in the walls of the stomach are arranged
transversely, longitudinally, and obliquely. The part of
the stomach next to the esophagus contracts and steadily
presses upon the contained food; but the posterior half
undergoes a series of wave-like constrictions. The result
is that the food is broken up into smaller masses and well
mixed with the digestive fluids. These movements have
been studied by feeding dogs and cats with bismuth sub-
nitrate, a drug which physicians often prescribe to allay
gastric irritation. This drug mixes with the food in the
stomach and renders the whole mass so opaque to the X-rays
that the shape of the stomach is easily seen with the aid of a
fluoroscope, and photographs can be made similar to those
often taken of the skeleton within the body.
From time to time the pylorus opens, allowing liquid
foods (often called chyme) to pass into the intestine. As
a rule, the large masses of solid food are held back in the
stomach.
The movements of the intestine consist of constrictions.
These sometimes appear in series at certain points, and then
next at the points which are halfway between the first con-
strictions. This causes a thorough mixing of the foods with
the digestive fluids. Another form of constriction which
occurs at intervals moves along the intestine and forces the
contents towards the large intestine. The above experiment
with rubber tubing illustrates this type of construction.
Such movements are known as peristaltic contractions,
147. Why Foods must be Digested. — We have seen in
our study of the frog that foods must be prepared so that
they may be absorbed through the lining membranes (epi-
thelimn) of the digestive organs into the blood. In other
words, foods must be capable of osmosing through mem-
branes; and most foods, taken into the mouth, are not
168 INTRODUCTION TO BIOLOGY
ready for this process. Glucose sugar and common salt dis-
solve in water, and are practically the only common foods
which can be absorbed without digestion. Even milk, our
most common liquid food, contains droplets of fat too large
for absorption which must be dissolved in preparation for
absorption. Moreover, milk becomes coagulated in the stom-
ach, and the clot must be digested.
Microscopic Examination of Milk, — ( D) Mount a drop of milk on
a glass object-slide. Examine (1) with low-power, (2) high-power
objective. Note the droplets of fat. These are lighter than the
water of the milk ; and, like oils in general, rise to the surface when
milk stands, forming the concentrated layer of fat which we call
cream. When the cream is shaken or agitated, as in a churn, these
fat droplets fuse together into larger masses of fat, which we call
butter.
Since digestion practically means the preparation of foods
for absorption, it is convenient to study the action of the
various digestive secretions (saliva, gastric juice, bile, and
pancreatic fluid) by adding them to foods in test-tubes ; and
after allowing time for a change, try for osmosis of the food
through membranes, such as fish-bladder or parchment.
There are good reasons for thinking that any food which
will osmose through a dead membrane will also be absorbed
under conditions which exist in the linings of the stomach
and intestine. Hence any digestive changes which will pre-
pare foods in test-tubes for osmosis will serve to illustrate
the changes which take place inside the living aUmentary
canal. But not all the digestive processes can be illustrated
by test-tube experiments, for the living digestive organs cause
some changes in foods which cannot be imitated in the lifeless
conditions in test-tubes. However, it is possible to perform
a number of experiments which will throw much light on the
various secretions of the aUmentary canal with regard to
HUMAN STRUCTURE AND LIFE-ACTIVITIES 169
their part in the digestion of the various kinds of foods.
We shall consider the digestive processes in the order
in which foods come in contact with the secretions in pass-
ing from the mouth-cavity through the esophagus into the
stomach, and thence into the intestine; this means con-
tact, in succession, with the salivary, gastric, intestinal,
pancreatic, and hepatic (liver) secretions.
148. Digestion by Saliva. — The fluid secreted by the
salivary glands consists chiefly of water and an enzyme
called ptyalin, which is very similar to the diastase of plants.
The dry ptyalin may be purchased from chemists, or saliva
may be collected in a test-tube (when one chews a piece of
rubber or gum it is secreted rapidly). The work of saliva is
the digestion of starch to a kind of sugar, which is absorbable.
This is illustrated by the following experiments: —
(D) Make some thin starch paste, by heating starch in water.
Notice that the resulting fluid is not clear, but opalescent. Place a
small quantity of the paste in a test-tube and add a few drops of
iodine solution. ^Note the color.
(D) Place some of the paste in a small bag made from a piece of
gold-beaters* membrane, or fish-bladder, and suspend the bag so
that it dips into water in a small tumbler or beaker. Or put some
of the paste in an osmose apparatus.* After allowing an hour or
two for osmosis to take place, transfer some water from the tumbler
into the test-tube, and add some iodine solution. Compare with
test of starch as directed above. Is there evidence that starch
osmoses through a membrane ?
(D) Along with the above, test grape-sugar or corn-sirup by
placing in water inside a bag or in the tube of the osmose-apparatus.
Wait the same time as in the case of the starch, and test water in the
tumbler with Fehling's reagent (§ 66). Does the sugar osmose?
(L) Mix some starch scraped from a potato in a drop of cold
water on a glass slide, put on cover-glass, and examine with low
power of microscope. Note the appearance of the starch-grains.
Now remove the slide from the mi<?roscope and heat slowly over an
* Described in ''Teachers' Manual of Biology," § 398.
170 INTRODUCTION TO BIOLOGY
alcohol- or a gas-burner until the water begins to steam. Examine
again with the microscope, and note the broken starch-grains. These
broken grains mixed with the water form starch paste, and the
opalescent appearance of the thin paste is due to the numerous parti-
cles of starch floating in the water. Examine starch scraped from a
baked or boiled potato.
(D) Place some thin starch paste in each of two test-tubes.
Add to one tube (No. 1) a few drops of clear saliva (filtered through
coarse filter-paper). Tube No. 2 has only paste. After twenty
to forty minutes note the appearance of the paste in the tubes,
especially with regard to opalescence. Pour a small quantity from
tube with the saliva (No. 1) into a clean test-tube, and add a few
drops of iodine solution. Is starch present?'
(D) Into another tube pour some starch paste from tube No. 1,
and into still another pour paste that has been acted upon by saliva
in tube No. 2. To each of the tubes apply Fehling*s test. Re-
sults? Conclusions? Do these experiments suggest why a dry
starchy cracker becomes sweet to the taste after being held in the
mouth for some time?
(D) If -time permits, prepare three tubes with thin starch paste.
To No. 1 add boiled saliva ; to No. 2 add normal saliva, and also a
few drops of acid (vinegar will do) to make the paste slightly acid
(test with blue litmus-paper) ; and to No. 3 add saliva, but keep the
tube standing in a tumbler fiUed with finely cracked ice. After
twenty to forty minutes test for starch and sugar as in above.
What do these experiments show regarding (1) effect of boiling
saliva, (2) effect of acid, (3) effect of low temperature ? Boiling
and low temperature have the same effect on all the secretions of the
stomach, intestine, and pancreas.
(D) In order to show that saliva changes starch into sugar capa-
ble of being absorbed (i.e., osmose through a membrane), take some
starch paste which has been acted upon (digested) by saliva for two
or three hours, place in a membrane bag or osmose-apparatus, allow
a half-hour for osmose, then test water in tumbler for sugar.
The above experiments simply prove that saliva digests
starch to a sugar; and that while starch does not osmose,
the sugar formed from starch does. Saliva, then, prepares
starch for absorption into the blood which flows in blood-
HUMAN STRUCTURE AND LIFE:- ACTIVITIES 171
capillaries beneath the lining membrane (epithelium) of the
alimentary canal, especially abundant in the stomach and
intestines. The sugar derived from starch osmoses through
the epithelium into the blood, just as it is demonstrated by
the above experiments that it will osmose through the dead
membrane of the osmose-apparatus.
It is evident that the above experiments do not prove
anything regarding the action of saliva on other nutrients.
If time permitted, we might try similar experiments with
proteins and fats, using these nutrients in place of starch,
and, of course, appl)dng the appropriate tests (§ 129). Such
experiments have been performed many times by physiol-
ogists, and their conclusion is that saliva has no digestive
power for foods other than starch.
No simple experiment will show that there is in saliva a
substance (ptyalin) which causes the digestion of starch, but
chemists have proved it to be present, and the cause of the
action of saliva. Moreover, it has been shown to be an
enzyme, because it acts by its presence, without entering
into the sugar formed, and a very small amount of it can
digest a large quantity of starch.
The amount of starch food which will be digested in the
mouth-cavity depends upon the length of time it is held
there. Hence, prolonged mastication is advisable. It has
been demonstrated within recent years that saliva continues
its action on starch for some minutes after the food reaches
the stomach, or until the acid gastric juice stops the action.
149. Digestion by Gastric Juice. — Physiologists have
found that the human stomach secretes from five to ten
quarts of gastric juice in twenty-four hours. This secretion
consists chiefly of water containing a small amount of hydro-
chloric acid and three enzymes — pepsin, rennin, and lipase.
The last named seems to be of most importance after food
172 INTRODUCTION TO BIOLOGY
reaches the intestines. The action of pepsin and of rennin is
most easily demonstrated. Pepsin may be extracted by
soaking stomach-membranes in glycerine, but the commercial
extract sold at drug-stores is most convenient for experi-
ments. Rennin is especially abundant in the stomachs of
calves, and is sold in grocery and drug stores under the
names of " liquid rennet " and *' junket tablets.''
Numerous experiments by physiologists have proved that
gastric juice digests proteins, curdles (coagulates) milk, dis-
solves some minerals in foods, and digests a small amount of
fat, but it has no effect upon starch. In this course, we have
time for only a few experiments which illustrate some of
these points.
The Action of Pepsin on Proteins, — (D) (1) Make some albumen-
solution by mixing white-of-egg in cold water, fill a test-tube half
full, add some dry pepsin or glycerine extract of pepsin, add enough
hydrochloric acid to make the mixture slightly acid to litmus-paper,
place the tube in a warm place near a stove or radiator, or in a " fire-
less cooker " or bucket of water heated to 37® C. (98 F.) and pro-
tected from cooling rapidly. After from three to ten hours, pour the
contents of the test-tube into a membrane bag or osmose-apparatus
and suspend in pure water. (Start the next experiment at this time.)
Allow an hour for osmosis. Test the water for proteins (§ 129),
using Millon*s reagent. Have proteins gone through the membrane
into the water?
(2) Pour some fresh undigested albumen-solution into a mem-
brane bag, and after an hour test for osmosis as in above experiment.
Does this albumen osmose? What conclusion is to be drawn con-
cerning the effect of pepsin and acid on the pjurticular protein used
in these exi>eriments?
If time permits, other proteins may be tested in the same way.
Try a hard-boiled egg grated or minced into fine particles, shreds of
lean meat, or cheese. Also, the acid may be omitted, in order to
show that pepsin alone will not digest proteins.
(D) The action of the rennin may be illustrated by adding some
commercial liquid rennet or junket tablets to milk. Place some of
HUMAN STRUCTURE AND LIFE-ACTIVITIES 173
the curds on filter-paper, wash with water, place in a test-tube with
some water, add strong nitric acid, heat. What nutrient is abun-
dant in the curds ? Pepsin and acid will digest them.
Summarizing, gastric juice can digest proteins, coagulate
the proteins of milk, and then digest them, and digest a
small amoimt of fat; but it does not digest starch. Fat
meat breaks up extensively when kept for some hours in
gastric juice, but only the protein walls of the cells have
been digested, allowing the contained oil to escape undigested.
Fat-Cells. — (Z)) Place a small piece of fat meat or suet in a drop
of glycerine on an object-slide, and with a pair of needles tease the
meat into as small particles as possible. Put on cover-glass, and
examine with low and high power of a microscope. Note the spheri-
cal cells filled with fat, which may be in the form of needle-shaped
crystals at the ordinary temperatures. If a small piece of fat meat
be soaked for a time in ether to dissolve the fat, and then placed
in iodine-eosin or other stains, the nuclei of the fat-cells will be
stained.
150. Digestion in the Intestine. — While the gastric juice
has power to digest protein foods, the fact is that most of
them do not remain in the stomach long enough for complete
digestion ready for absorption. At intervaLls, the muscular
ring of the pylorus relaxes and allows partially digested foods
to escape into the intestine, there to undergo final digestion
and absorption into the blood. This food which escapes into
the intestine from the stomach is water containing (1) much
starch not digested by the saliva, (2) sugar which was eaten
as such and much of the sugar formed by the digestion of
starch by the saliva, (3) dissolved proteins digested in the
stomach, (4) many particles of undigested proteins, and
(5) fat in liquid (oil) condition. The work of the intestine is
to complete the digestion of the proteins, fats, and starch ;
and to absorb the products of digestion into the blood and
lymph. •
174
INTRODUCTION TO BIOLOGY
151. Secretions in the Intestine. — These are formed by
the pancreas, the liver, and the glands of the intestinal walls.
The pancreatic secretion contains enzymes able to digest
any kind of food^ and consequently any
food not made ready for absorption while
in the stomach is digested by the enzymes
poured into the intestine. The final result
is that the starch, proteins, and fat which
enter the intestine undigested are prepared
for absorption. For this work the intes-
tine is specially adapted, because of the
numerous delicate processes (called viUij
singular is villiLs) which extend from the
lining membrane (see Fig. 60). These
are richly supplied with blood- and lymph-
vessels, into which digested food is ab-
sorbed rapidly.
152. Details of Digestion in Intestine.
— (Optional) The bile from the liver, the
secretion from the glands in the walls
of the intestine, and the pancreatic se-
cretion are alkaline, and quickly change
the acid food which arrives from the
stomach. Of the pancreatic secretion,
the enzyme called amylopsin acts like
the ptyalin of the saliva and digests
starch to sugar ; the enzyme called tryp-
sin acts on proteins much the same as
pepsin does in the stomach; and the enzyme lipase (or
steapsin) changes fats to glycerine and fatty acid. Glycerine
is absorbable. The fatty acid easily combines with the al-
kaline substances in bile and intestinal juice and forms a kind
of soap that is absorbed by the lacteals, which ar« exten-
F.Q. 60. Diagram
of two villi from
small intestine, c,
blood-capillaries; e,
epithelium cover-
ing villi ; gr, gland ; Z,
lacteal, a lymph-
vessel; t, connec-
tive tissue. (After
Hardy.)
HUMAN STRUCTURE AND LIFE-ACTIVITIES 175
sions of Ijonph-vessels into the villi. Since the pancreatic
secretion is able to digest rapidly the three kinds of food,
it is by far the most important digestive secretion.
Bile, from the liver, does not itself digest any food, but it
has been shown that fats are digested and absorbed better
when bile is present.
The intestinal juice, from the glands of the intestine, has
a number of enzjmaes of which the most important change
cane-sugar, milk-sugar, and malt-sugar into glucose or simi-
lar sugars.
(D) It is possible to demonstrate the effect of pancreatic secre-
tion upon proteins and starch by performing experiments similar
to the previous ones (§§ 148 and 149) with saliva and gastric juice.
See the ** Applied Biology," § 402.
163. Summary of Digestion and Absorption of Foods. —
Grape-sugar dissolved in water may be very slightly absorbed
in mouth-cavity and esophagus, some in stomach, but chiefly
in intestine. Cane and some other sugars are changed
before absorption in the intestine.
Starch. — Some digested to sugar (maltose) in mouth-
cavity and in stomach by saliva, but chiefly digested to
maltose in intestine by amylopsin of the pancreatic secre-
tion. Resulting sugar may be absorbed as stated for sugar
above.
Proteins. — Digested to absorbable form in stomach by
pepsin and in intestine by pancreatic secretion. Absorbed
to slight extent from stomach, but chiefly from intestine.
Milk proteins are coagulated in stomach by rennin as a special
preparation for digestion by action of pepsin, or later by pan-
creatic secretion.
Fats. — Chiefly digested in the intestine by lipase of pan-
creatic secretion, aided by bile and intestinal juice. Slightly
digested by lipase in the stomach. The protein part of
176 INTRODUCTION TO BIOLOGY
tissues containing fat (e.g., bacon or suet) is digested away
in stomach by pepsin, thus freeing the liquid fat.
Mineral foods. — Those that, Hke conmion salt, are soluble
in water require no change. Others present in various ani-
mal and vegetable foods which we commonly use are easily
dissolved in the acid gastric juice. All the mineral foods are
ready for absorption when dissolved in water.
154. Why Foods are Cooked. — The subjection of foods
to great heat in steaming, boiUng, baking, and roasting has
several useful effects : (1) The cell-walls and starch-grains of
vegetable foods are broken, thus preparing for action of
digestive fluids. Raw starch inside plant cell-walls is diffi-
cult to digest. (2) Parasites (such as tapeworms, trichinae
and disease-producing bacteria) are killed. (3) Easily-
digested gelatin is formed from the abundant connective
tissue of animal flesh ; and muscular and other tissues are
softened and loosened so that they can be readily acted upon
by juices in the stomach and intestine. Raw protein is
more easily digested than when cooked; but the last two
advantages named above make it desirable to cook meats.
(4) Most cooked foods are more palatable.
Beef placed in cold water and slowly heated loses to the
water certain substances, forming beef-tea. This has Uttle
food value ; but is pleasant and stimulating. Meat dropped
into boiling water coagulates on the surface and hence little
material is lost to the hot water. Roasting, broiling, boil-
ing, and steaming best prepare foods for digestion; while
frying in lard, butter, or oil causes the oily materials to
cover the food and thus render the penetration of digestive
juices more difficult. In general, a similar result comes
from any heating of starch and fat together; and hence
pastries, such as pie-crust, are more indigestible than would
be the same amount of the ingredients taken into the stomach
HUMAN STRUCTURE AND LIFE-ACTIVITIES 177
without heating together. Various methods of making
starchy foods porous, such as the action of baking-powder
and yeast, favor rapid penetration by the digestive juices.
166. Transportation of Digested Food to All Cells. — It
has been stated in a general way that foods are required by
all the living cells of the human body. The study of diges-
tion and absorption has shown how foods get into the blood
and lymph, and now we want to know how foods get into the
cells. This requires (1) that blood and lymph should be
moved from the capillaries in the walls of the digestive organs
to the capillaries in all the other organs of the body, and
then (2) absorption from the blood and Ijmaph by cells which
need food. Before one can understand how food is trans-
ported to all parts of the body, it is necessary to make some
study of the general plan of structure of the organs con-
cerned with the movement of blood and lymph, and also of
the nature of these two liquids. Hence our next lesson deals
with these topics.
BLOOD AND LYMPH
156. Structure of Human Blood. ^- If examined with the
microscope, blood is found to be a liquid, called plasma, in
which float numerous small bodies, blood-corpuscles or blood-
cells. About 90 per cent of the plasma consists of water,
which has been absorbed from the digestive organs, and
more than 9 per cent consists of absorbed foods. There are
also in the blood some excretions which have been absorbed
from cells and are on their way to the excretory organs
where they will be eliminated.
In human blood there are two kinds of corpuscles or cells
visible with the microscope ; namely, the red and the white
(Fig. 61). The red ones are bi-concave discs, about ^^^
of an inch in diameter. They are often seen in rolls like piles
N
178 INTRODUCTION TO BIOLOGY
of coins. The number of them is astounding, for there are
about five million in a cubic millimeter of blood. (A milli-
meter is approximately ^ of an
inch ; compute how many red cor-
puscles in a cubic inch of blood.)
In the diseased condition known
as ansemia, which is character-
ized by whiteness of the skin, the
, ^ * number of red corpuscles is greatly
Fig. 61. i.redblood-corpua- j j e> •^
des; iB, same in edge view; reduced.
S^n roUs; 4. white blood- Tj^^ ^.^j ^^j^^. ^j ^j^^^^ y^j^^j^
cells is due to a substance called
hcsmoglobirij which is most important in the blood's work
of carrying oxygen to the cells of the body.
The red corpuscles of man and other mammals have no
nuclei, except in embryonic stages; but all lower verte-
brates — birds, reptiles, amphibia, and fishes — have nuclei
in all red corpuscles, even in the adult animals. Scientists
have not yet found any reason for this difference.
It is also interesting to know that red corpuscles are con-
stantly being formed in the red marrow of bones.
(D) Obtain a long bone from a meat-market, break it open, and
examine the marrow.
The white corpuscles are irregular in shape, because they
move spontaneously like an amoeba
(§114). In human blood they occur in
the proportion of one white to 500 of
the red cells. They are exceedingly abun-
dant in blood taken from any inflamed
place, such as a boil, pimple, or wound. ^^^ ^^ various
Like an amceba, which they resemble, al- shapes of a white
though there is no connection as to origin, blood-cell,
the white cells can ingulf (" eat '0 small particles, such as
HUMAN STRUCTURE AND LIFE- ACTIVITIES 179
bacteria, by the flowing of the protoplasm around the ob-
ject. It is beheved that their work or function is that of
destroying bacteria and particles of dead cells. This ex-
plains why they are abundant in such places as boils and
abscesses. The material known as pus, which exudes from
such centers of inflammation, consists largely of white cor-
puscles and pieces of the dead cells of the inflamed tissue.
(D) Examine a drop of human blood spread on an object-slide and
protected with a cover-glass. A drop may be obtained by wrap-
ping a piece of string tightly around the second or third finger near
its end and then pricking the skin with a needle which is first made
sterile by passing several times through a flame. One small drop
of blood should be mounted on an object-slide and a second small
drop should be placed in a drop of normal salt solution (common
salt 7 grams in a liter of water, which gives 0.7 per cent). The red
corpuscles appear faint yellow in color when not in masses. They
have no nuclei. The rarer white cells are difficult to find, but may
be located by turning the micrometer screw about half a revolution
so as to throw the red corpuscles slightly out of clear focus, and then
a few white cells may be seen as glistening points of light in the field.
They are somewhat larger than the red cells.
Blood from a frog, fish, or bird, or permanent preparations of
these, are interesting for comparison with human blood. Nuclei
are in the red cells of these animals and the cells are much larger than
those in human blood.
Blood from an earthworm, crayfish, insect, or other invertebrate
animal has only white cells.
157. Coagulation of Blood. — Every one knows how blood
will flow from a cut for a time and then stop because some
of it has thickened into a jelly-like mass, which closes over
the cut and forms a scab. This process of thickening is
known in physiology as coagulation^ or clotting. Examina-
tion of blood allowed to clot in a drop of normal salt solu-
tion will show that coagulation is due to the formation of
delicate fibers which bind the corpuscles into a semi-solid
180 INTRODUCTION TO BIOLOGY
mass. These fibers are composed of a substance called
fibrin, which is a coagulated form of a protein called fibrin-
ogen. This is dissolved in the plasma of the blood and be-
comes hardened into threads when blood escapes from a
blood-vessel. If fresh blood of any animal be stirred with a
feather, the fibrin will form on the feather and the defibrin-
ated blood will not coagulate, thus proving that the forma-
tion of fibrin causes coagulation.
The use of coagulation is obviously to prevent excessive
bleeding. If blood did not coagulate quickly, a slight injury
to even a small artery might cause the death of animals, or
of men before surgical aid could be secured.
158. Lymph. — This is a liquid very much like blood, but
without red corpuscles. It is in part blood-plasma which
has passed through the capillaries into the very small spaces
which exist between the cells in all tissues. Thus lymph
comes into direct contact with the cells, giving to them food
and oxygen, which are dissolved in blood-plasma, and re-
ceiving from the cells some excretions, especially water.
These small Ijmaph-spaces between the cells are united
together into larger Ijmaph-capillaries, which in turn connect
with the large Ijmaph-vessels. These pour their Ijmaph into
the veins in which blood goes back to the heart. Lymph,
then, is in part plasma which passes from blood-capillaries
into Ijmaph-spaces, receives some water and other secretions,
then flows into Ijmaph-capillaries, and through larger vessels
back into the blood-veins near the heart. The arrangement
of blood and Ijmaph-vessels in any organ might be compared
with the irrigation systems for watering agricultural land.
The blood-capillaries correspond to the ditches from which
water seeps out into the spaces between particles of soil.
These spaces in the soil correspond to lymph-spaces, while
the lymph-capillaries and larger tubes correspond to the
HUMAN STRUCTURE AND LIFE- ACTIVITIES 181
drain tiles which carry excess water away from irrigated soil.
In short, the Ijmaph-spaces in any organ constitute a sort of
combined irrigation and drainage system for cells which are
not directly reached by the main canal system of the blood-
capillaries.
Lymph contains white corpuscles (Fig. 62), which are
able to pass through the walls of blood-capillaries into Ijmaph-
spaces. They are also formed in great numbers in the lym-
phaiic glands through which Ijmaph flows on its way back to
the blood in the large veins. In fact, the white cells in blood
are lymph-cells washed into the blood from the lymphatic
glands. The spleen and the tonsils are examples of large
l3anphatic glands, and there are hundreds of small ones in
various parts of the body.
CIRCULATION OF THE BLOOD
159. Need of Movement of Blood. — In order that blood
may serve its purpose as a distributor of food and oxygen to
the cells and as a remover of excretions from cells to the
excretory organs, it is necessary that it should be kept con-
stantly in motion. This is accomplished by the circulation
of the blood from the heart through the arteries and capil-
laries into the veins, which conduct it back to the heart.
That the blood thus goes around in a circuit was unknown
until 1621, when Dr. Harvey, of England, proved that in
man and other vertebrates the blood always flows in one
direction from the heart in blood-vessels, and that these are
so arranged that blood ultimately comes back to the heart,
I.e., makes a complete circuit. This circulation is remark-
ably rapid; it is believed that blood goes from the heart
through the capillaries in such a distant organ as a foot and
returns to the heart in about half a minute. (How many
times would the blood complete its circuit in a day ?)
182 INTRODUCTION TO BIOLOGY
160. Structure of the Heart. — (D or L) Procure from the mar-
ket a sheep's heart with the lungs attached, and with the membrane
{pericardium) surrounding the heart. Insert a large tube in the
trachea and inflate the lungs by blowing air into them. Note the
relation of lungs and heart. Carefully dissect away (with forceps
and scissors) the fat which adheres to the heart, taking care not to
cut off any arteries or the thin-walled veins. It is best to stuff the
veins with cotton or insert a small roll of paper.
The general form and external structure of the sheep's heart is
similar to that described and illustrated for the human heart in many
books on anatomy and physiology. Examine pictures in such books ;
note positions of the two auricles and two ventricles of the sheep's
heart. Notice that a probe (e.g., a blunt stick or glass rod) inserted
into a vein enters an auricle. The connection of the arteries with the
ventricles can be seen later (next paragraph). Cut across the heart
transversely about an inch from the pointed end (apex). This will
open the two ventricles. The left one is a rounded cavity, tlie right
is crescentic in outline. Note the relative thickness of the muscular
walls of the two ventricles. Now, take a blunt stick about the size
of a pencil, and inserting it into the left ventricle, probe carefully
until it emerges out of the largest artery. This is the a,orta, whose
branches are arteries leading to all the organs except the lungs.
Insert a similar stick into the right ventricle and out through its
artery. This is the pulmonary artery, whose branches conduct
blood to the lungs.
The action of the valves in the aorta or pulmonary artery in pre-
venting blood from flowing back into the ventricles can be demon-
strated on a heart with the apex removed, as follows. Insert a large
glass tube (about J" or J" caliber) into one of these arteries held
upright, and fill it with water. Or connect the artery with a large
funnel. If the valves are still in good order, the water will remain
in the tube or funnel.
To demonstrate the action of the auriculo-ventricular valves,
cut away the auricles from a heart with the apex removed, and then
plunge it, with the ventricle held downward, into water so as to float
the valves into the closed position.
Slit open the aorta to expose its valves (semi-lunar) ; and also cut
the side of a ventricle to show attachment of the auriculo-ventricular
valves.
HUMAN STRUCTURE AND LIFE-ACTIVITIES 183
161. The cause of circulation is the constant, rhythmic
beating of the heart, which is a muscular force-pump. Its
general plan of structure is illustrated by an ordinary rubber
bulb such as is used for atomizers and syringes. In such a
bulb there are two valves arranged so that when the bulb
is filled with water and then compressed, one valve (inlet)
closes and prevents the water from flowing outward,
the other valve (outlet) remains open and allows the water
to escape into the outlet tube. Then if the bulb be allowed
to expand, the outlet valve is closed by the back pressure of
the water in the outlet tube, and water rushing in from the
supply tube opens the inlet valve. The next compression
will again arrange the valves as first described above. (Draw
diagrams to illustrate positions of valves and with arrows
show direction of flow through a two-valved bulb.)
In a similar manner, there are valves arranged in the heart
in two places, one valve to prevent a flow of the blood back
into the veins (inlet) when contraction occurs, the other to
prevent the return flow from the arteries (outlet tube) when
relaxation or dilation takes place. And, as can be easily
demonstrated with a rubber bulb having one inlet and one
outlet valve, repeated contraction and expansion will cause
fluids to flow through always in the same direction, which is
determined by the arrangement of the valves. Hence, the
blood circulates because the heart is a pump with valves so
arranged as to force the blood to flow in only one direction,
out through one valve, and thence around through tubes
which lead back to the inlet valve.
There are valves (Fig. 63) in all but the largest veins and
they aid in the circulation of blood especially when contracting
muscles press upon the veins between two valves. The region
so pressed acts like a syringe-bulb which has outlet and inlet
valveSj each compression driving blood towards the heart.
184 INTRODUCTION TO BIOLOGY
162. Heart a Duplex Pump. — Like some pumping-
machines with two pumps united side by side, the heart of man
and of other mammals is double. To illustrate it correctly
with the rubber bulb mentioned in the above paragraph, it
would be necessary to place two bulbs side by side. One bulb
would represent the right and the other the left ventricle of
the heart. The right ventricle pumps blood into arteries
which extend to the lungs, and the left pumps blood to all
the other organs. Moreover, the blood pumped from the
Fig. 63. A, pocket-like valves of veins. Arrow pointing from C to H shows
valves opening towards the heart, while the lower figure shows how the
valve would float up and close if blood started to flow backward towards
the capillaries through which it came.
right side of the heart to the lungs comes back in veins which
connect with the left side, and the blood from the left ven-
tricle comes back to the right side of the heart. In other
words, the two sides are constantly supplying each other.
Both right and left ventricles contract at the same time, the
right one forcing blood into the lungs, and thence into the
left collecting chamber (left auricle), while the left ventricle
forces blood into all other organs, from which the blood re-
turns to the right collecting chamber (right auricle).
It is evident from the above that the two sides of the heart
must pump equal amounts of blood, for they beat (contract)
together, and each supplies the other with blood. Why then
should the walls of the right ventricle be;so much thinner
than those of the left ? The answer is that the right ventricle
HUMAN STRUCTURE AND LIFE-ACTIVITIES 185
meets with less resistance in pumping blood to the lungs
than does the left ventricle wtich pumps to all the distant
parts of the body. It is a familiar fact that a force-pump
that empties water into a near-by bucket is easier to work
than one connected with a long line of pipe or hose.
163. Pulse. — The well-known throbbing movement of
certain arteries (as in wrist and in temple) which are near
enough to the skin to be felt with the fingers, is due to wave-
like expansion of the arterial walls which begins when the
contracting ventricle forces its contained blood into the
already-full arteries. This causes expansion first of the
elastic aorta and then in succession of all the arteries, over
which a wave-like expansion passes as the amount of blood
equal to that discharged by the ventricle is moved onwards
towards the capillaries.
(D) It may be imitated with a soft rubber tube (5 or 6 feet long)
comiected to a small pump or syringe-bulb. While pumping water,
steadily press upon the tube with a finger, and note the pulsations
which follow each stroke of the pump. Then insert a glass tube with
a small opening into the end of the rubber tube, and pump rapidly
enough to make the jet of water a steady stream. Now note that
the rubber tube is somewhat expanded under internal pressure, and
that the " pulse " is still discernible.
This simple experiment imitates the conditions in arteries.
Their walls are elastic, the small arteries and capillaries exert
resistance and prevent the sudden flow of blood through
them (just as the small opening at the end of the rubber tube
does) ; the blood forced into the elastic arteries expands
them, and their recoil steadily forces blood through the
capillaries. The effect of the elastic walls of the arteries is
the same as that of a long rubber hose attached to a force-
pump, and of the elastic air compressed in the air-chamber
of pumps which Torce water directly into iron pipes. If the
186 INTRODUCTION TO BIOLOGY
arteries were rigid like glass or iron, there would be no pulse,
and blood would be sent id sudden spurts through the
capillaries.
(D) To illustrate this, replace rubber tube in experiment above
with a tube of glass of same caliber and length. (Any desired length
may be made by joining chemical glass tubing with short pieces
of rubber tubing.) When the pump is worked as before, jets of
water will issue from the end of the glass tube, no matter how large
or small an opening is present. The same thing happens in an ordi-
nary force-pump, and in order to get a steady flow of water in our
city water systems an air-chamber near the pump is used. (Explain
action of air-chamber.) Obviously the elastic arteries afford a
simple way to get the effect of an air-chamber on a force-pump ;
namely, a steady flow of fluid through the small branches (capillaries)
of the system of tubes. ^
The next section deals with still another advantage of
elastic arteries.
164. Regulation of Blood Flow. — It often happens that
an increased activity of the organs demands a greater move-
ment of blood for carrying more food, oxygen, or excretions ;
or a decreased activity demands a lessened circulation. In
short, there is need of a regulated blood flow from the heart
through the organs. This is brought al^out in two ways:
(1) by control of the heart, and (2) control of the arteries.
Control of the heart is through the nervous system, certain
nerves carrying to the heart impulses or " messages " which
increase the rate of its beat, while other nerves inhibit or
diminish the beat.
Control of the arteries is Hkewise the function of certain
nerves. The small elastic arteries in all living organs are
usually somewhat constricted, thus offering some resistance
to arterial flow, and keeping up a continuous stream through
the capillaries and veins back toward the heart. This con-
striction or reduced caliber is produced by a contraction of
HUMAN STRUCTURE AND LIFE-ACTIVITIES 187
the muscle-fibers in the walls of arteries, and this contraction
is under control of nerves.
When an organ needs an increased blood-supply (e.g.,
the stomach in digestion), it may be obtained by a nervous
reaction leading to a relaxation of the muscular walls of the
smaller gastric arteries, thus enlarging their diameter. Simi-
larly, the appUcation of hot water or mustard to the skin
leads reflexly to an increased blood flow; but cold water
produces the reverse effect of contraction of the arteries and
a reduced circulation of blood. (See bathing, § 208. Also
see account of heat regulation by the skin, §§ 197, 203.)
With the exception of the heart itself and the lungs and
brain, the larger organs of the body are well supplied with
nerves able to regulate the size of the smaller arteries and
consequently the amount of blood-supply.
Whenever there is a great increase in caliber of the blood-
vessels^ the heart beats faster in order to keep up the pres-
sure. In this way, certain drugs used by physicians may
increase the action of the heart by reducing the pressure in
dilated arteries.
USE OF FOODS IN THE CELLS
165. We have now studied the preparation and distribu-
tion of foods to the cells. Naturally we now ask questions
regarding the use which the cells make of the food supplied
to them. In studying the frog, we have noted that foods are
used in part for supplying energy and in part for repair and
growth. In order to apply this fact to human life we need
to consider more carefully the kinds of foods in relation to
the supply of energy and materials for repair and growth.
Are all foods equally valuable? Should we give any atten-
tion to the selection of our daily diet? Such questions are
188 INTRODUCTION TO BIOLOOT
common nowadays, and show that there is a widespread
popular interest in the uses of various foods. We shall first
consider foods for energy (especially heat and muscular work),
and later, foods for repair and growth. This is only a con-
venient division into topics, for some foods can serve all
purposes.
166. Energy and its Conservation. — Some general con-
siderations regarding energy will make clear ihe use of foods
as a source of power in the human body.
We are familiar with the fact that stored energy in coal
may be changed into mechanical energy by a steam-engine ;
and this into electrical energy by a dynamo ; and the elec-
trical energy into heat energy by an electric-stove, or into
light by an electric-lamp, or into mechanical energy by a
motor. Thus the stored energy of coal may be changed into
various other forms of energy which are known as mechanical
energy, heat, light, and electricity.
One of the important discoveries of the last century was
that in such changes no energy is lost to the world. At first
this statement will be puzzUng to one who has read that
85 per cent of the energy stored in a ton of coal does not
appear in the work of the best steam-engines in common use,
and hence seems to be lost energy. Also, there appears to
be a loss in transforming to electricity, for a 20-horse-power
steam-engine driving a dynamo cannot generate enough
electricity to drive a 20-horse-power electric-motor. How-
ever, this loss of energy from the practical standpoint is,
after all, not a real loss, for the same amount of energy
remains in the universe. This could be proved by placing
a small steam-boiler and its furnace, an engine, a dynamo,
and a motor together in a room with walls that would
prevent loss of heat. Then if some fuel were burned and
the machinery set to work, generating steam and electricity,
HUMAN STRUCTURE AND LIFE-ACTIVITIES 189
the explanation of the apparent loss of energy would be
found chiefly in the heat given off to the air in the room.
Adding this to the energy of the engine, dynamo, and motor,
the sum would equal the amount of energy which a chemist
can demonstrate by burning the same amount of coal in a
calorimeter (§ 168). In short, from 85 to 90 per cent of the
energy stored in coal is given off from the machinery as heat,
10 to 15 per cent appears in the mechanical energy of the
steam-engine, some less in the electrical energy of the dynamo,
still less in the energy manifested in the electric-lamp, heater,
or motor. But each apparent loss is represented by heat
given off to the surrounding air. The fact is, then, that
energy can be transformed from one form into another with-
out loss, or without reducing the amount of energy in the
universe ; that is, energy cannot be destroyed. The total
amount of energy in the universe is always the same.
Another important discovery is that energy is not
being created anjnvhere. The most wonderful machines
can do nothing but transform energy. For example, the
steam-engine is a machine for changing heat energy into
motion or kinetic energy. A water-wheel changes the po-
tential energy of water stored at an elevation into motion. A
watch changes into motion the energy stored in a wound-up
spring. An electric-battery changes the energy stored in
chemicals into electricity. And so we might go through the
whole list of known physical changes in the universe, and in
each case find evidence that transformation of energy, and
not destruction and creation, is constantly occurring in the
universe. Science has found no positive evidence concern-
ing either creation or destruction of energy in all its
forms.
The facts regarding changes of energy are now embodied
in the Law of Conservation of Energy f which is, that energy
190 INTRODUCTION TO BIOLOGY
can be transformed from one form into another, but cannot
be created or destroyed.
One of the most interesting phases of science study is that
of the changes of energy as presented in the science of physics.
The law of conservation of energy has numerous practical
applications in machinery. For example, a physicist would
never spend his time in trying to invent a " perpetual-
motion machine," for that would be completely opposed to
the established law of conservation of energy.
167. Energy of the Human Body. — This is manifested
externally, in the form of heat, and muscular and nervous
activity. The source of this energy is the stored or poten-
tial energy of food, and numerous experiments have demon-
strated that the law of conservation of energy applies to the
human body (and all other living things), just as it does to a
steam-engine. Food for the human engine and fuel for a
steam-engine are both necessary because they contain stored
energy. In both cases the original source of the stored
energy was sunlight, which green plants utilized in making
carbohydrates.
168. Foods for Energy. — The energy value of the three
important nutrients (proteins, carbohydrates, and fats) is
easily computed by chemists. Samples of such nutrients are
dried and then quickly burned inside an apparatus known as
a calorimeter (heat-measurer), and the heat thus generated
is measured in terms of the amount of heat necessary to raise
1000 grams of water one degree Centigrade. This amount
of heat is a caloric, the standard unit for heat measurements;*
and since heat may be converted into other forms of energy
(§ 166) , it is convenient for measuring the total energy of foods.
* Some authors of textbooks use the "small calorie," which is the amount
of heat necessary to raise one gram of water one degree C. It has an advan-
tage in measuring small quantities of heat less than enough to raise 1000
grams of water one degree.
HUMAN STRUCTURE AND LIFE-ACTIVITIES 191
The heat value of a gram of food measured iii this way is
as follows : for dry protein, about 5.6 calories (i.e., 5600 grams
of water 1°) ; for carbohydrates, about 4.1 ; and for fats,
about 9.3. It has been shown by a method described in the
next paragraph that a man or a higher animal obtains about
9 calories from fats, and 4 from carbohydrate foods ; while
protein is not completely oxidized in the living body, so that
one gram of protein gives only about 4 calories of the 5.6 it
contains. Obviously, proteins are not economical as foods
for heat or muscular energy, while carbohydrates and fats
give almost as much energy in the body as when they are
completely burned in a chemist's calorimeter. The chemical
result for fats and carbohydrates is the same in both cases,
for these foods produce carbon dioxide (CO2) and water
(H2O) in both the living body and the calorimeter.
Many substances which will produce heat energy in a
calorimeter could not be burned in an animal body. For
example, a gram of pure carbon (charcoal) gives 8.08 calories^
and a gram of hydrogen would give 34.5 calories ; but neither
of these could be oxidized at the temperature suitable for
protoplasm. This shows the importance of determining
whether foods give as much heat in an animal body as in a
calorimeter ; and this has been tested with a special appa-
ratus as follows : —
169. Calorimeter for Living Animals. — This consists of
a small room with walls arranged to prevent heat conduction
as far as possible, with coils of metallic pipes arranged so that
circulating water will absorb heat from the air in the room,
and with machinery for ventilation. Delicate apparatus
attached to the pipes for supplying air and water record
changes in temperature, and chemical analyses are made of
foods, breathed air, and excretions. There is also inside the
room a foot-power machine connected with a small dynamo
192 INTRODUCTION TO BIOLOGY
or otherwise arranged so that the man may treadle the ma-
chine and thus do easy, moderate, or hard work for eight
hom-s per day. Thus it is possible to determine the efifect
of different amounts of work upon food requirements.
Inside such special calorimeters men have lived constantly
for several days at a time; and chemists have carefully
compared the heat value of the foods oxidized with the heat
given ofif from their bodies to the air of the room and meas-
ured by the apparatus.
The results of many such experiments have shown that
foods are oxidized as stated in § 168 above. The apparatus
has proved of great value in cases where the value of a food
to a living animal or to man was unknown.
Many experiments with different combinations of foods
have led to the conclusion that men of about 150 pounds
weight engaged in sedentary work require food yielding from
2000 to 2400 calories per day of 24 hours. Men doing hard
physical work for eight hours per day require food furnish-
ing from 3000 to 4000 calories ; and very hard work may re-
quire more than 5000 calories. There are individual varia-
tions among men of equal weight doing the same work.
Some idea of the meaning of these figures will be gained from
the statement that 100 grams of protein food giving 400
calories, 100 grams of fat giving 900 calories, and 250 grams
of carbohydrates giving 1000 calories make a total of 2300.
This would suffice for a sedentary professional man or a
shoemaker, but a farmer, mason, or carpenter would require
more fat and carbohydrate food daily.
170. Foods for Muscular Work. — It was formerly sup-
posed that protein foods are necessary for muscular work,
and we frequently hear that " laborers must eat much meat
in order to get strength to work." This is not true. It has
been shown that muscular exercise does not require more
HUMAN STRUCTURE AND LIFE- ACTIVITIES 193
protein food, for fats or carbohydrates can furnish the neces-
sary extra energy. Two physiologists who used a mixed diet
of proteins and the other foods have proved that their
bodies oxidized as much protein on a day of rest as on a
day when they climbed a mountain over 6000 feet high;
and also that the amount of energy in the protein they
oxidized was not sufficient to Uft their bodies so high. Hence
the other foods (fats and carbohydrates) eaten must have
supplied the necessary extra energy. But if these men had
eaten more pure protein, they could have gotten from it the
necessary energy.
The conclusion from many such experiments is that large
amounts of protein are not needed for ordinary work. The
necessary amount is approximately the sarde for every day,
whether one is keeping quiet or working, and is from 60
to 100 grams of protein for a 150-pound man. Most well-
to-do people eat on the average more than 100 grams of pro-
tein in meat daily, and also they get much additional protein
in milk, eggs, bread, all vegetables — in fact, there is some
in most conmion foods.*
While the amount of protein foods needed daily is quite
constant for the ordinary adult, the amount of the other
foods should be in proportion to the energy required. A
student certainly needs less fats and carbohydrates than a
laborer does. So long as digestion is good, variation in
weight is a good index to the quantity of such foods needed ;
for if they are taken in excess and digested, there will probably
be increased weight due to storage in the fat tissues of the
body. Loss in weight by an adult who daily eats as much
* Per cent of protein in foods : lean meat 15-22 ; eggs 15 ; white bread 9 ;
dried beans 22 ; potatoes 2 ; peanuts 25 ; butter 1 ; oatmeal 16 ; olive oil ;
white sugar ; milk 3 ; green corn 3 ; fish 15-20 ; apples 0.4 ; rice 8 ; car-
rots 1 ; plums 1.
O
194 INTRODUCTION TO BIOLOGY
as 100 grams of protein indicates a need of more fats and
carbohydrates; and whether starch, sugar, or fats should
be used depends upon availability and digestibiUty of these
foods, and taste of the individual.
Growing animals and children require a higher proportion
of protein (see § 172) than do adults, who alone have been
considered in the above discussion.
171. Foods for Heat. — As previously stated, birds and
mammals must maintain a constant body temperature.
To a large extent they do this by preventing excessive loss
from the skin. The production of heat is in part the result
of muscular activity. We all know how easily we can keep
warm by exercise on a cold day, but how uncomfortably
cold we often feel if we simply eat our regular meals and
keep quiet. For this reason we require more clothing or
covering when sleeping, for then the circulatory and respira-
tory muscles and chemical changes in the digestive organs
are the chief sources of heat production.
More food is required in cold weather. The non-nitrog-
enous foods are most economically increased ; but whether
fats or carbohydrates should be used for increasing heat
depends upon their availability and digestibiUty. The in-
habitants of very cold regions can get fat from animals,
while those in temperate regions can store plant food, con-
taining large amounts of carbohydrates, for winter use.
When both kinds of foods are available, there is great indi-
vidual variation in taste for and digestion of carbohydrates
and fats ; and any desired combination of the two may be
made, provided that we remember the energy value of fats
as 9 calories, while sugars, starches, and proteins yield 4
calories per gram.
172. Foods for Repair and Growth of Protoplasm. — At-
tention has several times been directed to the fact that living
HUMAN STRUCTURE AND LIFE-ACTIVITIES 195
substance (protoplasm) is composed of protein. Also, it
has been stated that animal cells can make new protoplasm
only from protein foods, while plant cells can make protein
from the elements furnished by carbohydrates and the
nitrogen-containing substances absorbed from the soil.
There is abundant evidence that in the human body pro-
tein food is required for making new protoplasm in repair
and growth ; and that the foods lacking nitrogen (i.e., carbo-
hydrates and fats) cannot answer this purpose.
The amount of protein food required daily for repair of
the protoplasm in the cells of an adult man is believed to be
from 60 to 100 grams (28 grams to an ounce). However,
it should be said that some physiologists hold that 100 is
nearly twice the amount which is absolutely necessary ; but
many conservative authorities regard 60 grams of protein as
too little for a regular daily diet intended for repair and the
maintenance of good health.
If 200 grams of protein be eaten daily, the nitrogen ex-
cretion is double that from 100 grams. This indicates (1) that
protein is easily disintegrated in the living cells, (2) that
excess protein in the food is not stored for possible future
use, and (3) that if excess protein goes to cells they use the
necessary amount for making new protoplasm and disin-
tegrate into excretions the remainder, which may serve as
a source of energy. In other words, at least one-half of the
200 grams of protein might furnish heat or muscular energy
as other foods do.
173. Why limit Protein Food? — Since protein is neces-
sary for repair and growth, and may also supply energy, we
may properly ask why physiologists so often recommend a
limited protein diet, e.g., 60 grams per day. There are
several answers: —
(1) Protein as a source of energy is physiologically waste-
196 INTRODUCTION TO BIOLOGY
ful, for the human body can obtain only about 4 calories
per gram, which in the chemist's calorimeter shows 5.6
calories. This is because nitrogen excretion leaves the body
incompletely oxidized.
(2) Protein for energy-supply forces the body to handle
the useless nitrogen which it contains, for energy comes from
oxidation. of the carbon and hydrogen. A hard-working man
getting about 2880 calories from a daily diet of 120 grams of
protein, 100 of fat, and 375 of carbohydrates would have to
eat about 720 grams of protein daily to get the same amount
of energy if he took no food other than lean meat. This
would require the kidneys to excrete 200 extra grams of
urea which would come from the nitrogen content of the
extra 600 grams of protein. Such an excess of nitrogen ex-
cretions is injurious and tends to cause disease of kidneys
and other organs. Hence, it is better to get the necessary
energy from 100 grams of fats and 375 of carbohydrates
(100 X 9 4- 375 X 4 = 2400 calories) rather than from 600
of extra protein (600 X 4 = 2400 calories).
(3) Protein is not economical in a pecuniary sense. It
costs much more than common carbohydrates (starch in
vegetables, or even sugar) which have the same energy value,
namely, 4 calories per gram ; and clear lean meat is much
more expensive than fat meat or butter when we consider
that fat has 9 calories per gram, and hence has more than
double the energy value of protein, which has 4 calories per
gram.
174. Mixed Diet. — The three reasons given above have
led all physiologists to advocate a mixed daily diet contain-
ing (1) the protein necessary for daily repair, and for growth
in early life ; and (2) enough non-nitrogenous foods to fur-
nish the necessary heat and muscular energy. Whether these
foods for energy should be fat or carbohydrates depends
HUMAN STRUCTURE AND LIFE-ACTIVITIES 197
upon availability and digestibility. One man may digest and
use 100 grams of fat and 300 of starch as well as another
could use 50 grams of fat and 412 of starch ; but the energy
obtained is nearly the same.
Special foods are not known for special organs. That
celery is a " nerve " food, fish a " brain " food, lean meat a
" muscle " food, etc., are popular beliefs which are entirely
without scientific foundation.
It is not yet known whether proteins from plants are
equally valuable with those from meat. Certainly many
people have lived well on a vegetarian diet ; but those who
adopt it in its strictest form without milk, cheese, and eggs
should eat more than 600 grams of protein, because much of
the protein in plant tissue is not easily digested, and more-
over it is not known that the hmnan cells use all kinds of
plant proteins for use in repair and growth of protoplasm.
It also should be noted that with a strict vegetarian diet
there may be difficulty in getting enough protein without
an excess of carbohydrates, because there is relatively little
protein in many plant tissues. However, certain seeds (es-
pecially beans, peas, lentils, wheat, rye, and oats) contain
much more protein than the " green vegetables " (roots,
stems, and leaves of plants). Hence, it is possible by using
such seeds to make a strict vegetarian diet with proper pro-
portions of protein and other food. However, the safest
way for most people is to add milk and its products and eggs
to plant foods, if one has tastes or principles opposed to the
use of a Umited quantity of meats. On scientific grounds
there is no known objection to the proper use of meats, but
simply an objection to meats in excess of actual protein re-
quirements (i.e., above 60 to 100 grams a day).
References : Those who are interested in questions of diet should
read the chapters on "Nutrition," and "Hygiene of Feeding" in
198 INTBODUCTION TO BIOLOGY
Hough and Sedgwick's " Human Mechanism.*' Also obtain from the
Department of Agriculture, at Washington, the bulletins on the
nutritive values of foods.
OXYGEN-SUPPLY
176. Respiration. — This has already been defined as in-
cluding the functions of obtaining oxygen and eliminating
carbon dioxide. In some lower animals (e.g., earthworm)
the skin is the respiratory organ ; in fishes and others there
are gills; amphibians breathe with both skin and lungs;
but in the vertebrates higher than the amphibians lungs are
the sole respiratory organs. In all these cases the mem-
branes which take up oxygen also give out or excrete carbon
dioxide. For greater convenience in study, we shall in this
lesson confine our attention to the supplying of oxygen to
cells in the human body; and deal with the excretion of
carbon dioxide in the next lesson.
The respiratory organs consist of nasal passages, phar3mx,
larjmx, trachea (windpipe), bronchi (right and left branches
of the trachea), bronchial tubes (branches of bronchi), air-
chambers at ends of smallest bronchial tubes, diaphragm^
and wall of the thorax or chest-cavity.
176. Respiratory Passages. — We commonly think of the
nose as an organ for the sense of smell ; but the fact is that
only a limited amount of epithelium (lining membrane) in
some of the upper nasal cavities has nerve-endings con-
nected with the olfactory or odor-perceiving part of the
brain. Most of the cavities in the nose are respiratory, and
serve the purpose of warming the in-rushing air by contact
with the warm membranes ; and also the membranes and hairs
in the nose collect much of the dust from the air which enters.
The passages connecting those of the nose with the pharynx
are called post-nasal. They lie back of the soft . palate.
HUMAN STRUCTURE AND LIFE-ACTIVITIES 199
Into the same passages open the Eustachian tubes from
the ears. Some of the soft tissues in the lining of the post-
nasal chamber may enlarge and form adenoids (§ 199), which
interfere with free breathing through the nose and cause in-
jurious breathing through the mouth.
177. Structure of Lungis. — (D or L) Examine sheep's lungs ob-
tained from a meat-market. Notice the trachea and the arrange-
ment of its cartilaginous rings, incomplete on the side next to the
esophagus. Follow the two branches {bronchi) to the lungs. Try
inflating the lungs by forcing air through a large glass or wooden
tube inserted into the trachea. If it is found that one of the lungs
ha& been punctured, insert the tube into the bronchus of the unin-
jured lung and inflate. The free lungs can be inflated larger than
when they are inclosed in the ehest-cavity of the sheep. Now,
notice that when you cease blowing and allow air to escape through
the tube, the lungs quickly return to their original size. This is
caused by the elastic tissue in the lungs. Why does a rubber balloon
collapse when the air is free to escape ? The answer to this question
is the chief reason why air is expired or breathed out of living lungs.
How air is inspired or breathed in by living
lungs, will be considered in the next section.
The delicate outer membrane of the
lungs is the pleura, and similar tissue lines
the chest-cavity. These membranes are
lubricated by a secreted liquid which reduces
friction between the lungs and the chest-
wall during the movements of respiration.
Inflammation of these membranes, which
often occurs in connection with severe
"colds," is termed pleurisy.
^Cut into one bronchus and then split it
open as you work toward the smaller
branches. In this way follow up and lay
open a series of successively smaller branches
leading out toward the siu^ace of the lung.
The smallest branches end in air-chambers,
blood-vessels in the tissues of the lungs.
Fig. iS4. Ciliated epi-
thelial cells from the
trachea of a dog. The
hair-like processes on
ends of cells project in-
to cavity of the trachea.
They have constant
lashing movements.
(After Sharpey.)
Notice the numerous
The arteries can be
distinguished by walls thicker than those of the veins.
200 iNTBODUCTtON TO BIOLOGY
178. Breathing Movements. — Expansion of the chest-
cavity results in the external air entering through the trachea
into the air-chambers of the lungs.
Action of Diaphragm. — (D) Use apparatus constructed as fol-
lows : A sheet of dental rubber is stretched and tied over the base
of an open-top bell-jar, or lamp-chimney which is wide at base and
narrow at top. A short glass tube is tied into the mouth of a deli-
cate rubber bag {e.g., toy-balloon), and the free end of this tube is
then inserted through a hole in a cork which will fit tightly into the
top of the bell-jar. When the cork is placed, the rubber bag should
hang near the center of the jar. The glass jar represents the walls
of the chest-cavity, the glass tube represents the trachea, the rubber
bag stands for the elastic lungs, and the rubber sheet at the bottom acts
as. a diaphragm. Note that when this is made convex by pressure,
the rubber bag (imitating lungs) collapses. Why? Why is air
forced out of the toy-balloon when its tube is open? When the
rubber diaphragm becomes flat (imitating the downward or posterior
movement of the human diaphragm), the rubber " lungs " expand.
Why ? Why does water rush into a pump when the piston is raised ?
This apparatus illustrates the mode of respiratory action; but is
not exact, for the lungs fit and fill the chest-cavity, except that the
heart lies in a space between them.
The expansion of the human chest-cavity is due to breath-
ing movements, which are of two kinds : (1) those of the dia-
phragm, whose positions, due to muscular movements, may
be imitated by the rubber sheet across the mouth of the jar
used in the preceding demonstration ; and (2) to expansion
of the side-walls, increasing the diameter in a horizontal
plane. This latter is due to raising the ribs, and can be
demonstrated by measuring with a tape the circumference
of the chest before and after inspiration. Young children
expand the chest-cavity chiefly by the movements of the
diaphragm ; in many men the diaphragm does most of the
breathing work; in some women, particularly when tight
clothing is worn, the diaphragm is but little used, and ex-
HUMAN STRUCTURE AND LIFE-ACTIVITIES 201
pansion of the chest-cavity is due chiefly to movements of
the upper ribs. The ideal breathing movements combine
to use both ribs and diaphragm.
(L) Count the number of breathing movements per minute
(about 15 is usual). Do this first while sitting quietly, and later
after you have taken rapid exercise for a few minutes. Also, note
the movements of the abdomen and of the ribs when breathing
deeply and with loose clothing.
179. Changes in Breathed Air. — Analysis of air which
has been taken into the lungs and then expired shows that
it has lost oxygen. This means that the blood circulating
in the capillaries of the lungs has absorbed some oxygen from
the air in the air-chambers or air-vesicles around which the
blood-capillaries are arranged.
The lungs of the average adult man contain at the close
of a normal or quiet inspiration about 230 cubic inches of
air ; and about 30 of thi^ will be breathed out and exchanged
for as much fresh air. It is easy to remember that about
one-eighth of the air in the lungs is changed by each breath-
ing movement, i.e., about 15 times per minute when one is
breathing quietly. How many cubic feet of air will be
breathed in and out in 24 hours?
If a man takes a normal inspiration and then a forced ex-
piration (by '* blowing hard '0) about 130 of the contained
230 cubic inches of air will be expelled, leaving 100 that
cannot be forced out of the lungs. If a man breathes deeply
(forced inspiration), he can take in 100 cubic inches more
than the 30 of quiet breathing, making 130 taken in and
added to the 200 left in the lungs, total 330 cubic inches.
This is the full capacity of the human lungs at the greatest
possible distension. Of this 330, 230 may be expelled by
forced expiration. Obviously, a man trying to make a
record on a spirometer, such as are often used in gymnasia,
202 INTRODUCTION TO BIOLOGY
should first inhale deeply so as to get 330 cubic inches, and
then exhale forcibly so as to expel all but 100 that can never
be forced out.
The above figures are approximately correct for many
adults, but individuals vary greatly in lung capacity.
The advantage of frequent practice in deep breathing,
often recommended by physicians, is that the extra 100
cubic inches of air forced in will expand the lungs unusually
and thus allow fresh air to penetrate into air-passages which
are not properly aired or ventilated by the 30 cubic inches
ordinarily inspired.
180. Absorption of Oxygen by Blood in Lungs. — The
main blood-supply to the lungs is venous, from the right side
of the heart, which in turn receives most of it from all the
organs except the lungs, but the bronchial veins return a small
amount of blood to the right auricle from the lungs. This
venous blood passes through the capillaries of the lungs and
goes by way of the pulmonary veins to the left side of the
heart, thence to all the organs of the body ; but with only a
small amount via the bronchial arteries to their capillaries
in the lungs. On the way through the lungs the venous
blood is changed to arterial, by losing part of the carbon
dioxide and absorbing oxygen from air in the air-chambers
through the delicate walls of the chambers and into adjoining
blood-capillaries.
We must guard against the common error of supposing
that venous blood coming back to the lungs from all other
organs is devoid of oxygen. The fact is that every 100 cc.
of venous blood coming to the lungs has about 10 cc. of
oxygen dissolved in the blood, and when it leaves as arterial
blood it has about 20 cc. In other words, the amount of
oxygen is doubled in arterial blood. This additional oxygen
becomes combined with the haemoglobin in the red corpuscles,
HUMAN STRUCTURE AND LIFE^ACTIVITIES 203
and results in the change of color. Blood without the red
cells could not contain so much oxygen, and hence verte-
brates have a great advantage over the lower animals.
181. Distribution of Oxygen to Cells. — Blood carrying
oxygen received in the lungs passes from the left auricle to the
left ventricle of the heart and thence to all the organs, even
a small amount going back to the lungs in bronchial arteries.
As the blood flows through the capillaries in organs, some of
its oxygen is absorbed by near-by cells and some by the
lymph, which distributes it to cells that are distant from
blood-capillaries. RecalUng the statement that all living
cells are continually using oxygen, it is obvious that they
must demand constantly a fresh supply. If the blood were
to atop flowing for even a short time, the available oxygen in
every organ would soon be used by the oxidation going on
in protoplasm ; and the result would be asph3rxiation of the
cells. But under normal conditions the flow of blood is so
rapid that only approximately one-half of its contained oxygen
is extracted while it flows from the arteries through the capil-
laries into the veins. Hence, venous blood returning to the
limgs has about one-half the oxygen of the arterial blood
leaving the lungs (10 per cent in venous, 20 in arterial).
EXCRETION
182. Purpose of Excretion. — The products of oxidation
in the cells of the human body are of no further use, and when
accumulated may be injurious. Hence special organs have
as their work the elimination of excretions (1) from the
blood and lymph which absorbs them from the cells, and
(2) from the body. Most important of the excretions are,
as in the case of all other organisms we have studied, carbon
dioxide (CO2), water (H2O), and nitrogenous excretions.
The carbon dioxide is chiefly excreted by the lungs, the water
204 INTRODUCTION TO BIOLOGY
by the kidneys and some by the skin, and the nitrogenous
excretions chiefly by the kidneys.
183. Excretion of Carbon Dioxide. — Venous blood enter-
ing the lungs contains about 46 cc. of carbon dioxide in
100 cc. of blood, and the arterial blood leaving the lungs
has only about 40 cc. Thus it appears that approximately
one-eighth of the contained carbon dioxide is excreted as
blood flows through the capillaries in the lungs.
The chief difference between arterial and venous blood is
that the arterial has twice as much oxygen and seven-eighths
as much carbon dioxide. The small amount of nitrogen dis-
solved in the blood is always the same (1 to 2 per cent), for
free nitrogen takes no part in the activities of living cells in
animals.
It is incorrect to state, as do some old books, that " venous
blood is purified in the lungs " and that " arterial blood is
pure." The removal of only one-eighth of the carbon dioxide
does not make '* pure." We should not say that we have
made muddy water *' pure " if only one-eighth of the mud is
extracted. The words " pure " and " purify " should never
be used in connection with respiration of blood in the lungs.
It is easy to remember that blood doubles its oxygen and
loses about one-eighth of its carbon dioxide in changing from
venous to arterial blood in the lungs.
Carbon Dioxide in Expired Air. — (D) Connect a bellows, bicycle-
pump, or atomizer bulb to a glass tube, and blow air into some lime-
water (or barium- water). Note the effect. If much cloudiness
(precipitate) appears, compare with out-of-door air pumped directly
into the lime-water.
Now, exhale air from the lungs through a glass tube into lime-
water. Compare with the lime-water mixed with fresh air in above
experiment.
184. Excretion of Nitrogen and Water. — This work of
the kidneys is carried on by the numerous tubules in these
HUMAN STRUCTURE AND LIFE- ACTIVITIES 206
organs. A section of a kidney prepared for microscopic
study shows tubules cut transversely, longitudinally, and
obliquely ; and surrounding the tubules are abundant blood-
capillaries. The cells composing each tubule extract nitroge-
nous excretions and water from the blood, and then elimi-
nate them into the cavity or lumen of the tubule. The
water washes the excretion out of the tubule into the ureter
or kidney-duct, which in all mammals extends from each
kidney to the bladder. This is a reservoir for the temporary
storage of the excretions (urine) of the kidneys, and in all
mammals is connected with the exterior by a duct known as
the urethra,
185. Skin in Ezcretion. — Under conditions of high tem-
perature the human skin-glands eliminate water ('* sweat "
or " perspiration ") in which is dissolved salt and small
quantities of other substances commonly excreted by the
kidneys. However, all the water and other substances
could be excreted by the kidneys, and so the skin is not a
necessary organ of excretion. It will be explained in § 196
that the skin eliminates water in order to get rid of excess
heat, and hence excretion of water is not the primary work
of the skin-glands.
SUMMARY
186. Functions Serving the Cells. — (1) Digestion and
absorption of foods are functions for getting dissolved foods
into the blood, which directly or through the lymph dis-
tributes it to the cells of the entire body. (2) The lungs are
adapted to supplying the blood with oxygen, which is then
distributed to the cells. (3) The excretory organs (chiefly
lungs and kidneys) are adapted to remove from the blood the
various excretions formed in the cells. (4) The circulating
liquids (blood and lymph) serve the functions of food-supply,
206 INTRODUCTION TO BIOLOGY
oxygen-siipply, and excretion, in that food and oxygen must
be distributed to the cells and excretions taken away to the
excretory organs.
These four great functions, involving the organs of diges-
tion, respiration, excretion, and circulation, serve the cells.
We have noted that a one-celled organism, like an amoeba
or a Paramecium, does not need such a complicated mecha-
nism, foi* with respect to food, oxygen, and excretions it can
deal directly with the external world. The vast number of
cells in a higher animal has made necessary the complicated
organs of digestion, respiration, excretion, and circulation for
serving the cells.
NERVOUS ACTIVITY
187. Need of Coordination. — The functions which serve
the cells named in the preceding paragraph must work to-
gether or in harmony. For instance, if muscles are working
faster, there is need of more food and oxygen. This requires
more rapid circulation, greater oxygen-supply, and increased
digestion in order to supply and transport the necessary food
and oxygen. And faster work results in more excretions
and consequently greater activity of the circulatory and ex-
cretory organs in removing them. Thus, increased activity
of certain organs demands a corresponding increase in the
work of many others ; and there must be coordinated activity.
The function of coordination is part of the work of the
nervous system. This is parallel with the case of a frog's
functions ; but in addition the human nervous system has a
vast amount of other work arising from mental activities.
188. Reflex Action. — How the nervous system exerts its
coordinating power on other organs may be clearer after a
brief account of some simple cases of control.
If by accident one touches a finger to a hot stove, a sudden
HUMAN STRUCTURE AND LIFE-ACTIVITIES 207
contraction of the muscles will cause the hand to be jerked
away before the brain becomes conscious of the bum. The
explanation of such an action is that the hot stove stimulated
sensory nerve-endings in the finger, the stimulus was trans-
mitted to nerve-cells in the spinal cord between the shoulders,
and at once turned back as a. motor impulse, which, trans-
mitted along other nerve-fibers back to the finger, caused
the muscles to contract. Since the impulse originating in
the stimulated sensory nerve-endings appears to be reflected
back to the muscles by the spinal cord, the process is called
reflex action.
Headless frogs and other animals will make the same
reflex movements if their toes are stimulated. This proves
that reflex action is quite independent of consciousness
(knowing, feeling), for all observations on injured men and
animals indicate that the brain is the organ of consciousness.
For example, a man with the spinal cord seriously injured,
say in the middle of the back, would feel no pain in the legs
and could not move them voluntarily; that is, by conscious
action from the brain. Such facts, of which many have been
recorded in medical books, prove that there must be unin-
jured nerve-fibers connected with the brain in order to have
conscious control of organs. Hence, a headless animal or
one with the spinal cord cut off just back of the head could
not feel or be conscious of changes occurring in the body.
In the case of instantly closing an eyelid to escape a
threatened injury, the stimulation of the nerve of sight
(optic nerve) causes a reflex to muscles which move the eye-
lids.
The above are simple examples of imconscious reflex
actions which are constantly occurring. We learn to do a
large number of things reflexly, in addition to the funda-
mental processes such as breathing, heart-beat, digestion.
208 INTRODUCTION TO BIOLOGY
etc., which naturally and necessarily are subject to reflex
control. In walking, playing musical instruments, using
various tools, etc., we leam by long practice to act more or
less reflexly or automatically, and with little or no exercise
of the will (conscious control).
189. Conscious or Voluntary Action. — ■ The instance of
touching a hot stove, used above as an illustration of reflex
action, also affords an example of conscious action. Soon
after the hand is jerked away by reflex action, one becomes
conscious of being burned. Obviously, there are nerve-
fibers for transmitting the sensory impulse from the tip of
the burned finger to the brain. There a conscious action
may occur, and it may be reasoned that it is dangerous to
keep the hand anywhere near a hot stove, and so on, with
the result that it is decided or willed to take the hand far
away. This is accomplished by a conscious motor impulse
from the brain to the muscles of the arm, causing them to
move as the will dictates.
Moreover, the brain may also control the direction of the
movement ; that is, it may coordinate the contraction of the
various muscles. A splendid example of such coordinated
voluntary or conscious control of muscles is that of baseball
pitchers who can will to throw a ball to a given place and
at the same time consciously control the contraction of the
muscles of the arm so that the ball will be given the twirUng
or curving motion so much desired by experts in ball-playing.
Learning to do this by long practice means training the nerve-
cells in the brain so that they will come to control the muscu-
lar contractions and so cause the desired muscular movements.
190. Spinal Cord and its Nerves. — The human spinal
cord is usually described as lying in a cavity in the backbone
(vertebral column). Examination of some of the segments of
any backbone which may be obtained at a meat-market will
HUMAN STRUCTURE AND LIFE- ACTIVITIES 209
show that the cord lies dorsal to the central axis. More-
over, the cord is not completely covered by bone, and at
the uncovered places are the
spinal nerves. These are ar-
ranged in pairs (thirty-one
pairs all together). Each
nerve is divided near the
cord, and one branch of
root joins the dorsal side of
the cord, while the other
joins the ventral side. On
each dorsal root is a thicken-
ing called spinal ganglion,
which contains nerve-cells
whose fibers extend into the
cord and also out through
the nerves (Fig. 67). The
cells whose fibers constitute
the ventral root lie inside
the cord.
A cross section of a spinal
cord shows an X- or H-
shaped figure in the center.
In a fresh cord this is silvery
gray, and is called the gray
matter (Fig. 67). The sur-
rounding whitish tissue
{white matter) is chiefly com-
posed of connective tissue
and covered nerve-fibers.
Many of these fibers extend to the brain and others to the
regions of the cord where other spinal nerves are attached.
The gray matter contains many nerve-cells, especially those
Fig. 65. The white lines indicate the
general distribution of nerves from
the brain and spinal cord to all parts
of the body.
210
INTRODUCTION TO BIOLOGY
whose fibers extend out through the ventral
root into the spinal nerve and thence to mus-
cles and other organs.
The spinal cord is surrounded by membranes
which separate it from the hard walls of the
backbone. These contain numerous blood-
vessels, some of which extend into the cord.
Similar membranes are between the brain and
the skull.
Deep fm-rows or fissures extend longitudi-
nally on both the dorsal and ventral sides of
the cord and partially divide it into right and
left halves. The membranes which surround
the cord extend into these fissures and supply
the inner parts of the cord with blood-vessels.
It is known that the sensori/ nerve-fibers (e.g.,
those of touch in the skin) are connected with
the dorsal root of the spinal nerves, and that the
fibers which carry motor impulses from the cord
to the muscles and
other organs are in j^ /^ ,/r
the ventral root.
Since the dorsal and
ventral roots unite
into a spinal nerve,
it is clear that each
nerve is a bundle of
both motor and sen-
sory fibers, each one
connected with a
nerve-cell in a spinal
ganghon or in the
spinal cord.
A
Derve cell
from the
brain. ac,
axis cyliD-
der; c, cell;
t, terminal
fibers.
Fia. 67. Diagram of a cross section of spinal cOTd.
X-shaped gray matter in center. D, dorsal;
V, ventral; dr, dorsal root; vr, ventral root of
nerve; /, fissures; g, spinal ganglion; m, motor
fibers; «, sensory fibers; sn, .spinal nerve.
Dotted lines show paths of fibers from cells.
HUMAN STRUCTURE AND LIFE-ACTIVITIES 211
In reflex action (§ 188) the sensory impulse travels from
the skin to the cord by way of the dorsal root, and then in
the gray matter is '* reflected " to the motor cells whose
fibers leave the cord in the ventral root and extend out to
the muscles. How this " reflecting " is done is. unknown ;
we only know that events occur as stated. As a parallel
case, we know that electricity will '* travel " in certain sub-
stances according to definite laws; but scientists are very
much in the dark as to how and why it does so.
191. The Brain. — The human brain is a very compli-
cated mass of nerve-cells, fibers, blood-vessels, and connec-
tive tissue. The following brief account includes the most
important facts regarding structure of the brain.
The convoluted mass of the brain in the upper part of
the skull is the cerehrum, which is divided by a deep furrow
into right and left cerebral hemispheres. Beneath the back
part of the skull and just above the neck lies the next largest
part of the brain, the cerebellum. Beneath the cerebrum
are various small structures, such as the two olfactory lobes
from which nerves extend to the nose, two optic lobes with a
nerve to each eye, masses of white tissue through which
nerve-fibers connect opposite sides of the brain, several pairs
of nerves, and the &pinal bulb or expanded connection of the
spinal cord with the brain. Most of these structures can
be identified on a sheep's brain, obtained from a meat-
market, and hardened in strong alcohol or formalin solution.
Microscopic study of sections shows that most of the
nerve-cells of the brain are near the surface (cortex) y while
inside are the fibers connecting with the spinal cord and also
connecting various parts of the brain. Hence a cut across
a fresh brain shows gray matter (with nerve-cells) on the
outside and white matter (nerve-fibers) inside, just the re-
verse of the spinal cord.
212 INTRODUCTION TO BIOLOGY
The cerebral hemispheres are the center of mental life,
consciousness, and voluntary action. The cerebellum is the
center of coordination, causing muscles to work in definite
and controlled ways. The nerve-cells which control the
respiratory movements and the circulatory organs are in
the spinal bulb.
Localized centers in the cerebrum are known to exist. So
far our knowledge is limited, but there are areas in the sur-
face layers where the nerve-cells are concerned with special
organs; for example, there are such centers for muscular
movements concerned in speech, for hearing, taste, and smell,
sight, touch, and voluntary movements of various organs.
This discovery of localized centers in the cerebrum is already
valuable to surgeons. For instance, if an injury to a brain
interferes with any of the functions named above, the sur-
geon knows approximately where to look for broken blood-
vessels and other injuries.
The discovery of the centers controlling certain functions
has no bearing upon the once-popular pseudo-science called
phrenology, which pretended to locate certain general mental
powers by the contour of the skull. It was utterly im-
scientific because (1) there is no evidence of such localiza-
tion as phrenology claimed, and (2) there is much evidence
against the idea that the outer surface of the skull indicates
the degree of development of the brain beneath. •
ORGANS OF SPECIAL SENSES
192. Structure of Eye. — Use a hand-mirror and examine
your own eyes after reading the next two paragraphs. Refer
frequently to Fig. 68.
The center of the outer coat of the eyeball in front is the
transparent cornea^ through which one can look into the in-
HUMAN STRUCTURE AND LIFE- ACTIVITIES 213
tenor of the eye. All the remainder of the outer coat of the
eyeball is hard, white, and opaque (the sclerotic coat). One
might imitate the external appearance of an eyeball by paint-
ing a glass ball white, except-
ing a circular six)t to repre-
sent the transparent cornea.
Inside the cornea, and of
nearly the same size, is the
black, gray, or blue part of
the eye with a hole in the
center. The color pigment is
in a sort of thin membrane
(called iris)y and the hole in
the membrane is the pupil.
Just back of the pupil is the
UnSy which is bi-convex, trans-
parent, and elastic, so that by
pressure its shape can be
changed. Back of the lens
is the sensitive membrane {retina) , which is closely at-
tached to the back wall of the eyeball and hence is hemi-
spherical in shape. From near the center of the retina the
optic nerve extends to the brain.
The space between the lens and the retina is filled with a
transparent jelly-like substance {vitreous humor). The small
space between the lens and the cornea is filled with a watery
fluid {aqueous humor). If an eyeball y obtained from a meat-
market, be punctured, these humors escape and the eyeball
collapses. They are so transparent as not to interfere with
the passage of light from the cornea to the retina.
Between the retina and the outer white coat (sclerotic) is
a layer of tissue {choroid) with abundant blood-vessels and
black pigment. The pigment in the choroid and iris pre-
FiG. 68. Diagram of eye. a, axis;
6, blind spot; c, cornea; c^, choroid;
cm, accommodating muscle; i, iris; Z,
lens, m, directive muscles; o, optic
nerve; r, retina; «, sclerotic coat;
t>, vitreous humor.
214 INTRODUCTION TO BIOLOGY
vents light from entering the eye except through the pupil
and the lens. In the same way black pigment in a photo-
graphic camera prevents light from reaching the sensitive
plate or film except through the lens and the opening of the
diaphragm which corresponds in use to the iris and its opening.
In both a camera and an eye the whole structure is essen-
tially a light-proof box with a diaphragm or iris for regulat-
ing the amount of light, and a lens for focusing the rays of
light upon a sensitive plate or retina. The chief differences
between a camera and an eye are : (1) the eye is living
tissue ; (2) the sensitive plate or retina of the eye contains
living nerve-cells connected to the brain by nerve-fibers of
the eye-nerve or optic nerve; and (3) the lens of the eye is
focused, not as in a camera by moving the sensitive plate
nearer to or farther from the lens, but by muscles which
change the shape of the eye-lens. (4) There is always the
same " plate " (i.e., retina) in the eye.
The adjustment of the lens to suit diflferent distances of
objects seen is called accommodation. It is accomplished as
follows : The lens when not compressed is very bi-convex,
as may be seen in a lens cut from an eye obtained at a meat-
market. When an eye is resting in sleep or is looking at far-
away objects, the lens is much flattened (or made less con-
vex) by the pull of the elastic choroid upon the transparent
capsule which incloses the lens and attaches it to the choroid.
In order to see clearly near-by objects the lens must be
focused by being made more convex. This is simply the
elastic return of the lens toward its natural bi-convex shape,
and this return is permitted by a sheet-like circular muscle
which opposes the elastic pull of the choroid and thereby
eases the tension upon the lens.
It is evident from the above that the feeling of strain
when we look at very small objects is due to the pull of
HUMAN STRUCTURE AND LIFE-ACTIVITIES 215
muscles against the constant elastic pull of the choroid
upon the capsule that incloses the lens.
(D) In order to study the effect of change of shape of the lens
upon the focus of the eye, first set up a photographic camera and
focus upon near and distant ohjects by moving the lens. If one had
lenses for different distances (very bi-convex for near-by, and less
so for far-away) the distance from the lens to the sensitive plate
might be kept stationary in a camera. In the case of an eye the
distance to objects seen varies, and there is need of many lenses of
different curvatures ; or better still, of one elastic lens whose shape
can be changed to fit objects at any distance.
Eyes that cannot see distant objects clearly are said to
be " near-sighted/' and one with such eyes must hold print
very near in order to bring it into focus. This is due to the
lens being abnormally distant from the retina. Concave
glasses should be worn in order to change the direction of
rays of light and cause them to focus on the retina. Other
eyes are " far-sighted," and only by constant strain can the
lens be kept convex enough to focus the rays on the retina,
which is abnormally near the lens. Even when straining to
the utmost, some persons must hold books at arm's length
in order to read. Convex glasses correct such difficulty
and relieve the excessive strain. In old age the lens loses
some of its elasticity and fails to become convex enough
when trying to read, and hence a book must be held at a
distance, unless convex glasses are used. Astigmatism is a
very common congenital or inborn defect due to irregular
curvature of the cornea or lens, making it impossible to see
equally well lines which run in different directions, as on a
clock-face. For such eyes so-called " cylindrical " glasses
should be used constantly to avoid eye-strain.
193. The Ear. — The human organ popularly known by
this name is the external ear, from which a tube leads inward
216
INTRODUCTION TO BIOLOGY
to the tympanic membrane, or ear-drum. Beyond this is a
cavity known as the middle ear, and from it the Eustachian
tube leads to the pharynx. Still deeper in the head is the
inner ear, a complicated membranous structure lying in bony
cavities of corre-
sponding shape.
The lower part
(cochlea) is shaped
like the cavity in a
snail's shell and
the upper part has
three ring-like
canals (semi-cir-
cular canals).
Branches of the
auditory nerve con-
nect the sensitive
membranes of the
inner ear with the
brain.
The^ inner ear is
filled with a fluid.
Sound vibrations enter the outer tube, and throw the tympanic
membrane into vibration. This moves a chain of three bones
which extends across the cavity of the middle ear to the mem-
branous wall of the inner ear. Through this membrane vi-
brations are passed on to the fluid of the inner ear, and its
vibrations stimulate the sensory endings of the auditory
nerve which are sensitive to sound. The semi-circular canals
give us the sense of equilibrium, of which we are aware even
when we are blindfolded.
194. Smell, Taste, Touch, Temperature. — These senses
are connected with special nerves. Those of smell have
Fig. 69. Diagram of human ear. a, canal from
external ear; t, tympanum; w, middle ear
with small bones extending from tjrmpanum to
inner ear; s, one of the semi-circular canals of
inner ear; c, cochlea of inner ear; n, auditory
nerve; 6, bone surrounding middle and inner
ears; e, Eustachian tube, and p, its opening to
pharynx.
HUMAN STRUCTURE AND LIFE-ACTIVITIES 217
endings in the epithelium which lines certain upper cavities
of the nose. The nerves of taste end in the little projections
(papillce) on the tongue. Those of touch and temperature
are widely distributed in the skin of all parts of the body.
THE SKIN AND ITS WORK ^
195. Human Skin. — Microscopic preparations show that
the surface of the skin is made up of closely set cells, while
the lower side next to the muscles and bones is made up of
connective tissue (Fig.
70) . The cellular layer
is the epidermis, and
the connective tissue is
the dermis.
sm
(D) A piece of leather
tanned without the hair
will give a good view of
the intricately interlaced
fibers of the dermis, the
epidermis having been re-
moved in the process of
tanning. Soften such a
piece of leather by soak-
ing in hot water, and then
examine by tearing it
apart.
(L) The dermis is tied
down to deeper tissues
by connective-tissue fibersi
many of them elastic.
Pull up the skin on the
back of your hand, and note how quickly it returns to place when
released.
With a hand-lens examine the skin on arm or hand. Note the
delicate ridges and grooves, especially on the finger-tips. Press
Fig. 70. Diagram of section of skin. &,
blood-vessels; d, dermis; /, fat; h, hair;
n, nerves; ne, nerve-ending; s, sweat-glands
and their ducts; ac, homy layer of epider-
mis; sm, living layer of epidermis. {After
Wiederaheim,)
218 INTRODUCTION TO BIOLOGY
the finger-tips on an ink-pad and then make prints on paper. No
two persons have the same finger-prints; and so this has proved to
be a valuable means of identification, especially in records of crimi-
nals.
The surface of the skin looks scaly as seen under a strong
lens. These scales are dried cells. They swell in solutions
of caustic-potash or washing soda, and pieces scraped from
a calloused spot on the palm may be so treated in preparing
for microscopic examination. This swelling when wet ex-
plains why the hands get so soft when kept for a time in
water, especially soapy water.
A microscopic section shows that the scaly cells at the
surface are many layers thick, and that deeper down in the
epidermis are several rows of rounded or cubical cells, which
are usually brightly stained in preparations. These are
living cells, while the hard scaly ones at the surface are dead.
The scaly cells together constitute the homy layer, and the
lower cells of the epidermis form the Malpighian layer, or
living layer. The homy dead cells at the surface are con-
tinually wearing off, and the deeper living cells are by divi-
sion forming new cells, which are pushed toward the surface.
In warts, corns, and callouses the homy layer becomes
greatly thickened. The lower living cells form new cells
faster than the hard cells at the surface wear off. Friction,
pressure, and introduction of some foreign substance through
a cut or scratch are some of the well-known causes of these
thickenings of epidermis.
In the ordinary healing of a cut or bum, the cells of the
living layer form new cells to fill the gap beneath the blood-
clot or " scab " which forms on the surface. In extensive
bums it is sometimes necessary to resort to skin-grafting.
This means taking healthy pieces of skin from other persons
and applying them where the epidermis has been completely
HUMAN STRUCTURE AND LIFE-ACTIVITIES 219
destroyed. The cells of these healthy bits of grafted skin
soon become firmly attached and by repeated division grow
over the injm*ed surface.
Nails and hairs are specialized masses of homy cells. The
so-called " roots " of nails and hairs are deep-lying masses of
living cells which grow and divide rapidly. The pit in which
each hair is attached is called a hair-follicley and in it is a
conical elevation (papilla) from which the hair grows as cells
are formed and pushed outward. (HI glands (sebaceous
glands) open into the hair-follicles. Cutting or shaving hair,
contrary to popular belief, does not increase the number of
hairs, for the follicles are formed in embryonic life.
(L) Examine a hair with a microscope, and note that it is composed
of overlapping scales (cells). The center is also filled with cells.
The idea of barbers that hairs are hollow and require singeing to
prevent the escape of oil is absurd — but profitable to the barber.
Sweat-glands are abundant over the whole human skin.
They are most numerous on palms and soles, which also have
no hairs. Each sweat-gland is a tube extending from the
so-called *' pore " down into the dermis, where it is much
coiled and surrounded by blood-capillaries.
196. * Functions of the Skin. — Next to protection, the
most important function of the human skin is heat regula-
tion.
While the soft skin of frogs and other lower animals is im-
portant for respiration, the dry hard skin of mammals and of
man is of little use in this way. It has been shown by put-
ting a man in a rubber bag tightly fitted around the neck
that the limgs give off nearly two hundred times more car-
bon dioxide than does the skin. Hence, skin respiration is
of no practical importance to us.
Absorption by the skin is of little importance. Oily ma-
terials are often rubbed into the skin, and small quantities
220 INTRODUCTION TO BIOLOGY
appear to be absorbed. Probably the massage effect is most
important. It is known that some drugs can be absorbed
when thus rubbed into the skin.
Secretion of Swedt, — Under ordinary temperature condi-
tions the amount of secretion is not noticeable, because it is
evaporated as rapidly as formed. Drinking large quantities
of water raises the blood-pressure, and increases perspiration.
Certain drugs bring about dilation of blood-vessels and cause
profuse sweating ; others act in an opposite manner.
Normal sweat is nearly 99 per cent water. The one per
cent of dissolved material is chiefly mineral excretions
similar to those in the urine. If the skin were varnished, the
same excretion would be easily eliminated by the kidneys.
Sweat, then, is not necessary as an excretion, but as an aid
to heat regulation (see next section).
197. Skin as Heat-Regulator. — The fact that the notice-
able activities of the sweat-glands are usually associated with
great internal heat suggests that the chief function of these
glands is not so much to get rid of sweat (which the kidneys
could manage) as to discharge heat.
The muscular system is the chief source of increased heat
production, while the other organs probably generate the
heat which is more or less constant, as during sleep or com-
plete rest. When these organs are unable to supply the
necessary heat, shivering, which is involuntary activity of
muscles, may begin and thus increase the internal supply of
heat.
Some heat is given off in respiration. This is very impor-
tant for dogs, which perspire little except on the pads of the
feet. They " pant " when overheated, and lose heat rapidly
from the lining of the mouth and the surface of the pro-
truded tongue. Chickens are often seen breathing rapidly
with their mouths open ; and thus birds lose heat from their
HUMAN STRUCTURE AND LIFE- ACTIVITIES 221
lungs and air-sacs. In man, however, the skin is the great
heat-regulator.
The loss of heat from the human skin is controlled by
nerves, some of which regulate the flow of blood to the skin
and sweat-glands, and some stimulate these glands to
activity. Rapid exercise causes the sweat-glands to become
active. The eflFect of the sweat is illustrated by the fa-
miliar cooling of bottles of water wrapped with wet towels
exposed to warm air ; of water allowed to evaporate from the
hands or face on a summer's day ; or of the ix)rous water-
jugs and canvas bags which the inhabitants of some hot
countries use for their drinking water because the small
amount of oozing water is evaporated and cools the water
in the jug. We know also that following a bath on a hot
day the body cools rapidly, and the explanation is that the
heat of the skin was used to evaporate the water. Now, the
sweat-glands are simply mechanisms for covering the skin
with water ready for evaporation at all times when the skin
is warmed by overheated blood.
In addition to the heat lost by evaporation of water on
the surface, there is much loss by radiation, especially in
cold weather when the skin gets warm after exercise. Cer-
tainly at such times the warmer skin must radiate more
heat to the air, just as does a hot-water pipe when the tem-
perature of the circulating water increases.
The importance of sweat evaporation as a method of cool-
ing the body depends upon the temperature and humidity
of the surrounding air. Dry cold air takes up very little
watery vapor, while dry warm air takes up much water.
Hence in hot weather loss of heat by the evaporation of
sweat becomes more important. Obviously, hot dry winds
would favor evaporation ; while hot moisture-laden air pre-
vents rapid heat loss both by radiation and by evaporation.
222 INTRODUCTION TO BIOLOGY
This is the usual condition on oppressively hot days when
heat prostrations and sunstrokes are common. The moist
hot air prevents proper loss of heat from the skin, and the
internal temperature rises too far above 99° F., which is the
best temperature for human protoplasm. Obviously, reduced
heat production will help to prevent excessive internal heat ;
arid this means keeping as quiet as possible and eating
sparingly, in order to reduce the activity of muscles and the .
digestive organs. (Why do people in tropical climates with
hot sun and moisture-laden winds suffer from heat more
than do the inhabitants of equally hot but dry regions?)
Fever is due to increased production of heat (caused by
toxins of disease), and is usually accompanied with dry skin
and inactive sweat-glands ; but occasionally even great per-
spiration does not discharge the heat fast enough. The so-
called " wasting '' indicated by loss of weight during prolonged
fever is due to the rapid oxidation of tissues when little or
no food is available. Drugs aflFect fevers by reducing oxida-
tion in tissues or by promoting perspiration and consequent
heat loss from the skin.
The eflFect of cold baths upon heat regulation by the skin
is considered in § 208.
CHAPTER IX
BIOLOGY APPLIED TO PERSONAL HYGIENE
198, Hygiene. — In this chapter it will be shown that
very many of the facts and principles of biology are of great
value when applied to making the human body freer from
disease and a more efficient mechanism for carrying on a use-
ful and happy life.
That department of biological science which deals with the
established principles of human health is commonly known
as hygiene. It is simply a phase of applied biology. When-
ever hygiene deals with the health of individuals, how to take
care of oneself, as in rules for eating, breathing, sleeping, etc.,
it is called personal hygiene. This is chiefly the principles
of biology, particulariy of physiology, applied with a good
proportion of common sense. By this we mean, for example,
that one who h£is studied the structure and working of the
lungs will need only conmion sense to enable him to see that
tight clothing will interfere with the natural movements, and
then the hygienic rule " do not wear tight clothing " is seen
to be a scientific application of the biological principles re-
lating to human breathing. The same is true with regard
to every organ ; and we do well to examine every proposed
new rule of hygiene from the standpoint of applied biology.
Thousands of foolish rules of hygiene have been published,
but it is usually possible to select the good ones because
they are obviously based upon the principles of biology.
We should keep a sharp lookout for such applications of
223
224 INTRODUCTION TO BIOLOGY
biology to the unscientific rules of personal hygiene which
so many people accept without question.
Public hygiene or sanitation means principles of biology
apphed to increasing the health of a conmiunity of people.
Problems relating to clean streets, pure food-supply, infec-
tious diseases, sewerage, and water-supply are aspects of
public hygiene with which health officers are concerned.
RESPIRATORY ORGANS
199. Habits of Breathing. — The eflFect of tight clothing
is referred to in the preceding section. All authorities in
medicine and hygiene unite in condemning the fashion of
wearing any clothing which interferes with breathing move-
ments.
Exercise in deep breathing is important, for it trains the
respiratory muscles so that fuller expansion of the lungs
occurs regularly. This means that fresh air goes deeper
into the air-tubes. Consult your physical-training teacher
as to the advantage of training in breathing for athletics.
Breathing through the nose is the natural way, because
the air is properly warmed and much dust is stopped in the
nasal passages. Mouth-breathing is abnormal, but very
common. Children who breathe habitually through the
mouth probably have enlarged adenoids in the upper part of
the pharynx back of the soft palate. If not removed at once
by a competent surgeon, they may seriously interfere with
the health, cause deafness, or even deformation of the
upper jaw. They commonly disappear after childhood, but
then the damage has been done. Hence any special diffi-
culty with natural breathing through the nose should be
referred at once to a doctor for advice.
200. Ventilation. — Frequent renewal of the air of build-
ings is required because the breathing of human beings and
PERSONAL HYGIENE 225
the combustion in stoves, lamps, etc., use oxygen and add
carbon dioxide, and because bacteria are carried in the dust
of the air. Moreover, human breathing adds moisture and
raises the temperature ; and this is chiefly the cause of the
discomfort one feels in crowded churches and theaters.
Electric fans simply keep the air moving inside an over-
heated room and do not ventilate. All systems of ventila-
tion should provide for the exit of the lighter warm air near
the ceiling and the entrance of fresh air (preferably warmed
previously by heaters) near the floor. At night when the
body is properly protected in bed, the supply of fresh air
may safely be very cold. The important rule is to get sis
much fresh air as possible without chilling any part of the
body by drafts. That is far more harmful than poor venti-
lation, for in this way "colds" are often produced (§ 203).
In order to have good ventilation it is not necessary that
there should be a noticeable current of air in a room.
For accounts of the best methods of ventilation, see books
of hygiene and household science. The subject is so exten-
sive that only the great principles can be suggested here.
201. Avoiding Dust. — Owing to the fact that dust often
carries dangerous bacteria and that there is a harmful effect
of accumulated dust. in the lungs, dust in the air should be
eliminated as far as possible from homes, factories, and
public buildings. Old-fashioned sweeping and dusting are
dangerous unless windows are opened and the wind allowed
to blow out the dust and bacteria. Carpet-sweepers and
especially vacuum-cleaners are better. Damp cloths should
be used for wiping dust from furniture ; brushes and feather-
dusters are relics of the dark ages and should never be used.
If brooms must be used indoors, damp sawdust will help
keep dust from rising into the air. The most sanitary modem
dwellings have no carpets which are not easily taken outdoors
Q
226 INTRODUCTION TO BIOLOGY
for cleaning ; and the floors are painted, varnished, or waxed
so as to make washing easy and sweeping unnecessary.
202. Artificial Breathing. — This means causing the lungs
to respire after normal breathing h£is been stopped by drown-
ing or by gas asphyidation. One of the best methods of
artificial respiration is as follows : The subject is laid on the
groimd in the prone position with a thick folded garment
under his chest. The operator kneels beside the subject
(or athwart him), facing the head, and places his hands on
each side over the lower ribs. Then the operator slowly
throws the weight of his body forward and downward, and
thus presses upon the thorax of the subject, which forces
air out of the lungs. Then the operator gradually relaxes
his pressure, but without removing his hands. These move-
ments are repeated regularly at the rate of twelve to fifteen
times a minute imtil normal respiration begins, or until all
hope of normal respiration is given up. Many have been
restored to life after such movements have been continued,
for more than an hour. For other methods, see special chap-
ters on " accidents " in books on hygiene. The teacher
should give practical lessons on this important topic, select-
ing one student as patient and another as operator.
203. Colds in Respiratory Organs. — Severe " colds "
which are liable to lead to bronchitis (inflanmiation of bron-
chial tubes), pleurisy (inflammation of pleura of lungs), or
pneumonia (congestion of blood, with certain poisonous
bacteria in the lungs) require medical advice. Chronic con-
gestion of nasal membranes leading to the condition known
as catarrh should receive medical attention, for a slight opera-
tion or special treatment may be necessary to effect a cure.
For preventing colds see § 208 on bathing. One whose
skin has been chilled should hasten to restore the normal cir-
culation of the skin by exercise, friction, hot drinks, hot bath
PERSONAL HYGIENE 227
(with great precaution against more chilling), and in extreme
cases by certain drugs which physicians advise. It is dan-
gerous to neglect a " cold," especially in its incipient stages.
DIGESTIVE ORGANS
204. Teeth. — The proper care of the teeth is commonly
emphasized in the books on " physiology " written for ele-
mentary schools, and need not be repeated here. The fact
that bacteria are largely responsible for dental decay suggests
the daily use of antiseptic tooth-powders and mouth-washes.
206. Hygiene of Eating. — The practicable hygiene of
digestion for most people is that which suggests the time
for eating, and the amount and kind of food.
The question of the time of meals depends upon other
physiological demands. A light breakfast and a light lunch
are best for busy people whose muscles or nerves are hard-
worked. The best time for dinner, the chief meal of the
day, is after the day's work and a brief rest. The reason
for this is that intellectual activity leads to a marked increase
in the amount of blood in the vessels of the brain, and phys-
ical work affects muscles similarly. The result must be a
withdrawal of blood from the vessels of the digestive organs
and a consequent retardation of the digestive process.
The above plan suits most people ; but there are numerous
individual exceptions, and each one must experiment with
himself if he would find the most satisfactory times for meals.
Work and habits of human beings vary so greatly that there
can be no universal law of eating. Certain rules for general
application have been established by the experience of thou-
sands of people, and probably most important for remember-
ing are the following : (1) There should be regularity in
meals. (2) Physical and mental fatigue interfere with di-
gestion. (3) Overeating acts in a similar way.
228 INTRODUCTION TO BIOLOGY
The selection of diet and its amount is similarly variable.
Hard physical exertion and exposure to cold demand abim-
dant food for energy, and the question of easy digestibility
is of minor importance. On the other hand, persons of
sedentary habits should avoid unnecessary amounts of all
kinds of foods ; and also observe well their own peculiarities
as to digestibility of carbohydrates and fats.
Eating anything between meals is, as a rule, inadvisable;
but here again there are individual exceptions, and knowledge
of the possible harm to digestion will lead to caution.
Some water (not iced) should be taken at every meal, for
it is needed in liquefying the contents of the stomach in prepa-
ration for escape into the intestine. But water should not
be taken at the same moment with solid food, for it " washes
down " the food and thus prevents mastication. The old
idea that, water dilutes the gastric juice does not now seem
to be very significant, for it has been discovered recently
that water soon passes from the stomach into the intestine
and gastric juice is secreted rapidly.
Overeating is the chief point on which many people need
physiological advice. Scientific studies have often shown
that men need no more food than stated in § 166, and yet
a large number of people take more daily. Especially is it
true that we use too much protein (§ 173), thus unnecessarily
overworking all organs without any gain. The other foods
are also used to excess by many whose daily activities do not
require so much stored energy; and the conmion result is
fat-storage, which often becomes so excessive as to be un-
comfortable or even a danger to the heart and other organs.
In the majority of cases, excessive fat-storage is due to the
overeating of foods containing carbon, hydrogen, and oxygen
(sugar, starch, butter, and fat meat). The over-fat condition,
once established, is difficult to change; and hence yoimg
PERSONAL HYGIENE 229
people should guard against an excessive increase in stored
fat due to intemperate eating.
An excess of meat diet is more harmful than an excess of
other foods, for the reason that most proteins eaten are
quickly oxidized and excreted within a day. The nitrogen
excretions formed from excess proteins may play an important
part in the development of gouty and rheumatic conditions.
The value of mastication has been the subject of much
discussion, and is still uncertain ; for there are some people
who masticate little and have perfectly healthy digestion,
and there are others who masticate extensively and claim to
have thereby cured indigestion. The truth is that it is
largely a question of the kind and amount of food and the
habits of the individual. Certainly one who eats an excessive
amount of starchy food, or who has starch-indigestion in the
intestine, will do well to masticate starchy food and allow
the saliva to exert its digestive influences as long as possible,
for thereby he may cure a form of indigestion. But this does
not prove a rule for all people or for all articles of diet ; and
with no more than the necessary amount of properly cooked
food the average individual can safely follow his natural
instincts as to the amount of mastication. There are those
who point to the rumination of cows and sheep for evidence
that naturally animals masticate food for a long time; but
this gives no rule for human guidance. In the first place,
a cow's natural food is uncooked and otherwise unprepared ;
second, her digestive organs are quite unlike the human;
and third, dogs more closely resemble man in structure and
in foods and they never masticate. Obviously, it cannot be
concluded that man should chew his food long because cows
and sheep do. The only scientific conclusion must be based
upon individual human experience; and this indicates that
some people with weak digestion of starch neffed to give
230 INTRODUCTION TO BIOLOGY
special attention to mastication, but that most people may
safely forget their jaws while eating (i.e., masticate instinc-
tively) provided that they do not eat too rapidly or in excess.
However, it is well for each person to experiment upon him-
self, and thus determine how far special attention to masti-
cation is important for himself.
One point in favor o^ thorough mastication deserves men-
tion, namely, that it tends to prevent overeating. When food
is rapidly swallowed, there may be an excess taken before the
gastric nerves give us warning of too much food. Prolonged
mastication tends to avoid this result, possibly because the
sugar obtained from the salivary digestion acts upon the
gastric nerves just as sweets before a meal " take away the
appetite " or reduce it. In this way prolonged mastication
helps one apply the rule that eating should stop before hun-
ger is completely satisfied.
Stimulants, — Should digestive stimulants be avoided, is
a much-discussed question, usually with regard to alcoholic
drinks. It also should apply to spices, condiments, coffee,
tea, cocoa, carbonated water, and even hot food ; for these
all have some stimulating effect upon the digestive organs.
It is often said that such stimulants are not natural, for ani-
mals do not require them ; but it may be answered that ani-
mals do not lead sedentary lives, undergo intense nervous
strain, or otherwise interfere with proper digestion. Cer-
tainly we can often gain by the temperate use of some stimu-
lants; but harm will always come from overstimulation,
which is most likely to happen in the use of tea and coffee
(§ 230) and alcohol (§ 218). Probably the most useful and
safest of all stimulants are hot foods and hot drinks, such as
hot bouillon, hot milk, or even hot water.
206. Psychology of Digestion. — We have already defined
psychology as the science of the mind ; and here it is impor-
PERSONAL HYGIENE 231
tant to note that the mind greatly influences digestion. It is
well known that the thought, sight, or smell of savory food
causes the " moutH to water," that is, stimulates the salivary
glands; and there is a similar effect on the gastric glands.
We know also how certain disagreeable mental states may
cause loss of appetite and even nausea. In fact, the gastric
glands may fail to secrete, and indigestion and other dis-
turbances may be caused by the direct influence of the
mind.
Such well-known facts suggest the importance of pleasant
surroundings while one is eating. Here is the secret of the
good digestive influence of music, jolly company, and other
things which make for a happy state of mind during the meal
hour. That is a time when every one should drop his cares
and troubles.
On the other hand, there is a certain real danger in pleas-
ant accompaniments of meals. It is simply that fine fare
and surroundings which are agreeable to all the senses tend to
feasting or overeating, a result often capable of unhealthful
consequences. The only safety is in learning self-control so
that one's stimulated appetite may not lead on and on to
gluttony.
SKIN
207. Skin Cleanliness. — The scientific reason for clean-
ing the skin, especially by the use of soap and warm water, is
that it removes dirt, which is objectionable primarily be-
cause it is unsBsthetic and therefore disagreeable to refined
people, and secondarily, because dirt contains micro-organ-
isms which may produce disease. In fact, dirty hands have
caused typhoid and other dangerous diseases by leaving
bacteria on dishes and on foods. Formerly it was supposed
that a third reason for cleansing the skin with soap was to
remove substances which " clog the pores of the skin and keep
232 INTRODUCTION TO BIOLOGY
in excretions '' ; but the skin has little to do with excretions
except as water is incidentally eliminated while the skin is
reducing the internal heat; hence " keeping the pores open "
is a weak argument for cleaning the skin. The fact is that
the pores " open '' quickly, even on the dirtiest skin, when
exercise develops excess of heat. However, we do not need
the unscientific theory of " keeping pores open,'' for the
aesthetic and bacteriological reasons named above are suffi-
cient to convince any civilized person, and especially any one
who sees the force of the rule, " use soap because refined
people do so and the barbarians do not." This is more sen-
sible than trying to show that soap is necessary for hygienic
reasons, for there are healthy people who never or rarely use
soap. Such health is not surprising, for soap has very much
to do with aesthetics and little with health, except in possible
bacterial infections.
The abuse of soap by many refined people deserves atten-
tion. Exposed parts of the body must be washed very fre-
quently with soap, and preferably with warm water ; but a
complete bath with soap and warm water is taken too fre-
quently by many persons. Soap removes the oily secretions
and the warm water dilates the blood-vessels of the skin,
increasing liability to colds. A warm soapy tub-bath once
a week should be followed by cold water which causes con-
traction of the blood-vessels (§ 414).
As far as is consistent with cleanliness, soap with cold water,
and especially cold water without soap, should be used daily
on all parts of the body not exposed. The special reason
for cold instead of warm water is given in the next paragraph.
208. Bathing as a Skin Tonic. — Above we have considered
bathing for cleanliness only ; but here we are interested in
bathing for health. While only in removing bacteria does
skin cleanliness appear to be necessary for health, we have
PERSONAL HYGIENE 233
fortunately a very strong argument for daily bathing as a
means of leading to healthy action of the skin blood-vessels,
and indirectly of the whole body. For this purpose water
should be much colder than the temperature of the body,
and is best applied as sponge-bath, shower-bath, or plunge-
bath (as in sea-bathing). Contact with the cold water first
causes a reflex action leading to a reduced caliber of the skin
arteries, and the skin quickly becomes pallid. A reaction
follows brief exposure to the cold water, and the blood-vessels
expand, the skin glows, and the bather feels stimulated. No
such good effect comes from too long exposure, or in some
cases when the water is too cold.
The explanation of the value of this cold bathing is that
it gives the skin practice in readjusting its blood-supply when
exposed to a low temperature. Many persons, especially
those of sedentary habits, have skins which are not accus-
tomed to react quickly to changes of temperature ; and hence
if chilled their skin arteries remain contracted, blood which
ought to circulate in the skin is congested in some internal
organ, inflammation develops, and a " cold '^ follows. Fre-
quent practice in readjusting the blood-vessels, as given
by cold bathing, will make the skin more likely to react and
continue to receive its fair share of blood whenever exposed
to low temperature, thus tending to avoid the dangerous
internal congestions known as " colds." It should be noted
that " colds " are not confined to the lungs and respiratory
passages, for one may have an unrecognized " cold " from
congestion of blood in other organs, especially in stomach,
intestine, and kidneys.
If in spite of systematic cold bathing one's skin some-
times gets so chilled that it does not soon react, the normal
circulation should be restored by ways suggested in § 203.
To allow the skin to remain chilled for hours is dangerous.
234 INTRODUCTION TO BIOLOGY
Cold baths are best taken before breakfast, never within
two hours after a meal, because the rush of blood to the skin
interferes with the proper supply in the digestive organs.
209. Skin Foods, Hair-Tonics, Cosmetics. — All the much
advertised " skin foods " are fraudulent, for only digested
and assimilated foods supplied by the blood can build up
the skin cells. Some of these foods are composed of epsom
salts and coloring matter, the salts doing no good to the skin
and the dye being possibly harmful. Others are chiefly
colored vaseline or lanolin whose value can be no more than
lubricants like pure vaseline at one-tenth the price of " skin
food.''
There are conditions when the scalp is made more healthy
by tonics which reputable physicians prescribe; but the
claims for most patent hair-tonics are absurd. At most, a
hair-tonic can do nothing except correct diseased conditions
of the scalp. Most people who use hair-tonics have no need
of scalp medicine and they waste money in attempting to
grow hair when they have a constitutional tendency towards
thin hair.
Some of the patent hair dyes contain poisons. Various
preparations for bleaching the skin, removing freckles, etc.,
are widely sold. Many of them are fraudulent and some
of them dangerous. One complexion bleach which sells at
$2 per bottle is borax in orange water and costs the manu-
facturers six cents to make. Some of the face bleaches con-
tain poisonous compounds of mercury.
NERVOUS SYSTEM
210. Overwork and Sleep. — Unhealthy conditions of the
nervous system are frequently the result of overwork, both
mental and physical. Hence it is important that brain-
workers should consider the hygiene of the nervous organs.
PERSONAL HYGIENE 235
Regular mental work, as well as physical work, should be
Ihnited to a number of hours per day ; and these should be
the hours before late in the afternoon, when the maximum
exhaustion of nervous force occurs. Nervous exhaustion
from mental overwork is most often due to neglect of this
rule; and the brain-worker should limit his regular day's
work to a reasonable nmnber of hours per day, and those
when the brain is at its best. Too often mental overwork
simply means such long days at intellectual tasks that exer-
cise, recreation, and sleep are neglected. Sooner or later,
this means the inevitable penalty of nervous disturbance, if
not serious breakdown.
There come times in the lives of many brain-workers when
some important work demands temporary nervous strain;
but the man who is wise in the laws of hygiene will try to re-
duce the necessary strain to the minimum and to follow it
with as much recreation as possible.
The importance of fads and avocations as a means to men-
tal and physical recreation is great. Every man and woman
should cultivate at least one hobby. Even collecting postage-
stamps, coins, and natural-history specimens may be made an
important daily relief to a nervous system tired by the reg-
ular day's work ; but best of all avocations are those which
are as far different as possible from the regular work, e,g,, a
greenhouse, a garden, or a work-shop for a man engaged in
mental work.
But overwork is not all due to excessive exercise of the
nervous organs directly, for physical work may lead to ner-
vous disturbance. This is obviously due to the fact that
muscular contraction occurs only as the result of nervous
action. Moreover, there may be the added effect of the
wearisome monotony of uninteresting toil.
The close relation of muscular and nervous work points
236 INTRODUCTION TO BIOLOGY
to the important hygienic law that mental work should not
be forced after physical exhaustion, or physical work after
becoming mentally tired. The time for hard physical exer-
cise is not near the close of a day of such intense mental
strain that the tired nervous system seems to rebel at lash-
ing the muscles into action. That is certainly a time to
rest or recreate in any way which is not approached as an
unpleasant duty. Conversely, the time for hard study is not
at the close of a day of exhausting physical work. Whether
mental work may safely succeed physical activity, or vice
versa, usually depends upon whether one finds it possible to
take up the change of work without constantly goading one-
self against a feeling of exhaustion. Ambitious workers will
not meet with the one great danger in this advice ; namely,
that of confusing real exhaustion and mere laziness.
Sleep, — Probably more important than any other rules
of hygiene are those concerning sleep. Loss of sleep does
far more damage than starvation. Men have voluntarily
fasted 30, 40, and even 50 days, and afterward quickly re-
gained their normal weight and health; but it is certain
that a normal healthy man could not go without sleep so
long. Sleep is a period of rest, repair, and growth, and is
especially important for growing children. Much of the
physical harm to children in crowded tenements is due to a
combination of late retiring to rest, uncomfortable bedrooms,
disturbed sleep, and early rising. Many a child in the poorer
regions of great cities is anaemic, nervous, haggard in face,
listless in school, sluggish at play ; and the real trouble may
be that he is not getting enough good sound sleep. This is
likewise true of some adults, who should average seven to
nine hours of sleep in each night. As far as possible, this
should be taken during the quietest hours (10 p.m. to 6 a.m.)
when the external stimuli tending to cause awakening are
fewest.
PERSONAL HYGIENE 237
MUSCULAR SYSTEM
211. Exercise for Health. — Those whose business or
pleasure leads them to sedentary habits of life need to con-
sider most seriously the question of physical exercise. The
scientific reason for exercise is to be found in the coordina-
tion of muscular activity with all the other organs of the
body, rather than in the development of the muscular system
itself. In short, most people should exercise primarily in
order to get reactions of the digestive, circulatory, respirar
tory, and nervous organs, while secondarily and incidentally
they may develop their muscular systems. However, it is
very doubtful, in the opinion of many qualified physiologists,
whether excessive development of the muscular system is
best for the general health of those who are not professional
athletes or laborers. The well-known aphorism, *' Mens
Sana in corpore sano '' (a sound mind in a sound body),
means that giant intellects ought to be located in healthy
bodies in which all the functions are properly coordinated,
and it should not be understood to mean that only one with
the muscles of a champion athlete can hope to do great
intellectual work. On the contrary, it is a remarkable fact
that some of the greatest work in literature, art, and science
has been accomplished by men and women who suffered
from lifelong physical weakness. In such examples there
is hope for all who are physically weak by nature. Athletic
constitutions commonly originate congenitally and not in
gymnasia.
The average man and woman, then, should exercise for
health, deriving it from renewed activity of the organs that
are closely coordinated with muscles, and from rest and
recreation for the brain. This reference to the association
of exercise and recreation is important, for we certainly
238 INTRODUCTION TO BIOLOGY
derive most benefit from exercise which is at the same time
pleasurable and recreative. Herein is one great advantage
of many forms of outdoor exercise over gymnasimn work.
212. Excessive exercise is not beneficial when it leads to
exhaustion ; and severe over-strain may lead to injury of the
heart, blood-vessels, or lungs. Athletic enthusiasts often
answer this medical criticism against certain extra-strenuous
games, such as football and rowing, by claiming that the
moral gain from severe athletic contests overbalances the
recognized danger of great physical harm from excessive
exhaustion. That there is moral gain worth while in com-
pelling tired muscles to obey to the point of exhaustion is
extremely doubtful ; and those who are fond of quoting that
" Waterloo was won on the playing-fields of Rugby '' should
re-read history and note the victories in peace, and even in
battles scarcely less strenuous than Waterloo, won by men
whose moral fiber was certainly not directly traceable to
previous athletic training on any school-playground. Moral
qualities which make men great are inherent, not originated
by any one form of activity ; and hence we are not justified
in excusing dangerously excessive exercise because one or
more famous generals or other great men happened to play
football, or some other game, when they were boys. There
is no scientifically proved moral effect of physical strain
which in the slightest degree militates against the hygienic
rule that exercise for health should never be carried to ex-
treme exhaustion. The world would have far more healthy
and efficient men if this rule for muscular activity were more
often applied both in work and in play. ,
PERSONAL HYGIENE 289
PHYSIOLOGICAL EFFECTS OF STIMULANTS AND
NARCOTICS*
213. Introduction. — Man has long been more or less
accustomed to take into the body certain substances (alcohol,
tobacco, tea, coffee, certain drugs, etc.) which are not prop-
erly classed with the ordinary food-materials, for their value
as sources of energy and materials for repair is so slight as to
be negligible. All these substances are conveniently grouped
imder the heading " Stimulants and Narcotics," which
indicates that their action in the human body is either to
excite or stimulate greater activity of certain organs, or to
reduce their activity and tend to produce stupor or sleep
(narcosis). Both the exciting action of stimulants and the
quieting effect of narcotics are pleasurable to most people,
and it is solely for this peculiar pleasure that mankind has
adopted the habit of using the various substances which
afford stimulating and narcotic effects.
214. Are Stimulants and Narcotics Needed ? — It is in-
teresting to note that no animal naturally makes use of any
of the stimulants and narcotics ; and hence it is often argued
that the human species ought to be natural and avoid them.
However, this is a rather weak argument, for in many other
ways man has ceased to be natural (e.g., cooking food is
certainly unnatural for animals), and with . advantage to
himself. Clearly the use of stimulants and narcotics must
be judged by their good or bad effects upon men, and not
rejected simply because animals do not use them. The
experience of animals indicates that man does not absolutely
need stimulants and narcotics ; but it has no bearing what-
ever on the question of whether man may or may not profit-
*To Teachers: See "Teachers* Manual of Biology," § 463, for notes
concerning the use of this section.
240 INTRODUCTION TO BIOLOGY
ably make use of such substances. This will be discussed
at various places in the following sections.
216. Examples of Stimulants and Narcotics. — Alcohol
in small quantities appears to be a stimulant which increases
the activities of many organs. It is well known that alcohol
in large quantities produces a narcotic effect, leading to the
complete stupor of intoxication. Many physiologists believe
that even a small quantity of liquor containing alcohol has
a narcotic, sedative, or depressing effect on the nervous
system and through this influences other organs. Opium is
well known as a powerful narcotic which quiets active organs,
and in large doses leads to a fatal sleep. Most users of
tobacco in any form claim that it has a soothing effect, i.e.,
is a narcotic. Tea and coffee contain substances which are
usually stimulating to most persons. Many drugs used by
physicians (e.g., strychnine, nitro-glycerine) are powerful
stimulants, and are given in exceedingly small quantities.
When powerful narcotics are demanded as relievers of pain,
physicians commonly use opium and its products (lauda-
num, morphine).
216. Alcohol and Common Alcoholic Fluids. — The
formation of alcohol from sugar will be described later in
§§ 279, 280, which deal with fermentation caused by the
yeast-plant. Practically any natural substance which con-
tains starch or sugar may undergo fermentation. Thus
juices expressed from grapes, apples, and other fruits, and
the carbohydrates in grains of rye, corn, and barley, and in
potatoes, are commonly used in producing alcoholic liquors,
of which the chief varieties are mentioned below.
Malt liquors (beer, ale, and porter) are made from malt,
which is made from sprouted barley grains. These are ground
in water, and allowed to ferment. Hops are added and give
a bitter flavor. Such a fermented liquor consists chiefly of
PERSONAL HYGIENE 241
water, 1 to 8 per cent alcohol, and small quantities of other
substances derived from the grains used.
Wines are juices of grapes which have fermented and pro-
duced 6 to 12 per cent of alcohol. Some wines are stronger
(15 to 25 per cent) because brandy or strong alcohol has
been added when bottling.
Distilled liquors (whisky, gin, brandy, rum) contain 30
to 50 per cent alcohol, and are made so strong by distilling
the fermented fluids (water with rye, corn, oats, or potatoes
for gin and whisky ; molasses in water for rum ; wines for
brandy). Various flavoring and coloring materials are added
to the distilled liquors. They differ essentially only in color,
flavor, and proportions of alcohol. Some of the substances
used to color and flavor are harmful, but are used in such
small quantities that the alcohol is chiefly responsible for the
physiological injury commonly done by the distilled liquors.
Alcohol in a more or less pure state can be made by re-
distilling and otherwise purifying any fluid in which fermen-
tation has occurred. Since distilled liquors are nearly half
alcohol, it is easily obtained from them. The grain alcohol
of commerce is usually from 91 to 95 per cent pure; i,e,j it
contains 5 to 9 per cent of water. A special quality for
scientific purposes is about 99 per cent pure, and is very
expensive to make.
Commercial alcohol is usually called " grain-alcohol," or,
in chemical terms, ethyl alcohol. Wood-alcohol, or methyl
alcohol, is commonly made from wood, and when taken into
the human stomach is very poisonous. In all the discus-
sions in this lesson, the word *' alcohol " is used to mean
grain or ethyl alcohol, for this is the characteristic constit-
uent of the alcoholic liquors whose physiological effects are
under consideration. The effect of alcoholic liquors is largely
in proportion to the amount of contained alcohol, and so it
242 INTRODUCTION TO BIOLOGY
is justifiable and convenient to deal directly with the effects
of alcohol and to neglect, temporarily, the minor fact that
alcohol, as commonly taken in wine, beer, whisky, etc., is
diluted with water and variously flavored.
217. Is Alcohol a Poison? — In popular usage, the word
" poison " is associated with such powerful substances as
arsenic, strychnine, snake-venom, and others which, when
introduced into the human body, produce marked and even
fatal disturbances. In scientific usage the term is applied
to many substances which cause demonstrable disturbance
of any function of the body. There is no substance which
is always a poison, for even strychnine and ricin may be
diluted so as to produce no noticeable disturbance. A cup of
coffee is not poisonous to an average adult, and yet it con-
tains a greatly diluted dose of caffein, which in large amounts
is a poison. Tea, coffee, ginger, pepper, and many other
things taken with food contain small quantities of sub-
stances which in large amounts are poisons. Even common
salt in very large quantities has proved a fatal poison. Evi-
dently the word " poison " has a relative significance, and
involves the quantity. In general, we apply it only to sub-
stances which in very limited quantity are harmful. The
question, then, " Is alcohol a poison? " can be answered only
by reference to the amount of alcohol and to the constitution
of the individual who drinks it. That alcohol in large and
intoxicating doses has proved fatally poisonous is well known,
and that it commonly produces profound disturbances of
various organs when used excessively and habitually is also
common knowledge; but whether alcohol in very small
quantities is a poison, as we commonly understand the
word, is a difficult scientific question which only physiol-
ogists can answer by experimental studies made with ani-
mals and men.
PERSONAL HYGIENE 243
A liter (nearly a quart) of whisky, gin, or rum given in one
dose would kill any animal weighing 67 kilograms. (How
many pounds ?) Evidently alcohol in large quantities is a
poison. However, even a victim of the alcoholic habit
would not drink a quart of whisky within a short time.
The question is whether in the smaller amounts, such as are
commonly used by drinkers, alcohol is a poison. The next
four sections discuss this question with reference to the
organs of digestion, circulation, and respiration, and the
nervous system ; and it is pointed out that it is impossible
to show by scientific methods that ordinary small amounts
of alcohol produce effects comparable to those of the sub-
stances which druggists label " poison." Hence it must be
concluded that, so far as we now know, alcohol in small
amounts is not harmful enough to warrant labeling it
" poison." We do not so label common salt, although a
strong solution taken into the stomach has caused death;
and while coffee contains a dangerous poison, a pot of the
beverage should not be labeled as dangerous. Most people
would be misled by such a label, for they know well that in
ordinary quantities common salt and coffee produce no symp-
toms of poisoning. Likewise, in very limited quantity alco-
hol is not a poison in the sense that we understand various
drugs to be poisons. However, while alcohol is not a poison
to be labeled as druggists label the common poisons, it
produces very harmful effects on human organs. Nothing
is to be gained by saying that alcohol is a poison when we
mean that it is injurious.
218. Effect of Alcoholic Liquors on Digestion. — When
taken into the stomach, alcoholic fluids cause a marked in-
crease in the flow of gastric juice from the glands of the
stomach wall. There is also an increase in the amount of
the constituents of gastric juice; namely, pepsin and hydro-
244 INTRODUCTION TO BIOLOGY
chloric acid. Wine, alcohol, beer, whisky, brandy, and mix-
tures of these stimulate the gastric glands in this way. The
alcohol quickly leaves the stomach, being absorbed into the
blood, leaving the gastric juice in concentrated form.
Whether such an effect of alcoholic fluids upon gastric se-
cretion is directly harmful or not seems to depend upon the
amount of alcohol present. Thus strong beverages, like
brandy, gin, and whisky, with 40 to 50 per cent of alcohol,
retard the digestive action of pepsin on proteids ; but this
effect depends upon the amount of alcohol taken, the amount
of food in the stomach, the strength of the gastric juice, and
the health of individuals. Hence it is impossible to lay down
any absolute rule as to the minimum quantity of alcohol
which will harmfully affect digestion in the stomach. It is
certain, however, that large drinks of alcoholic beverages do
impede gastric digestion even in healthy individuals; and
since the alcohol does no particular good, it is wisest not to
run the risk of unhealthful effects upon digestion.
It should be emphatically stated that the effect of alco-
holic drinks upon digestion is not solely due to the amount
of alcohol in them. Thus sherry wine with 20 per cent of
alcohol retards digestion much more than does an equal
quantity of 20 per cent pure alcohol. Large amounts of
claret wines have a similar, but less noticeable, effect. The
same is true of ale, beer, and other malt liquors. When any
of the wines and malt liquors are used freely with meals,
there is likely to be a considerable retardation of the diges-
tive processes.
Concerning the effect of moderate amounts of alcoholic
fluids upon gastric digestion, it appears from experiments
made by competent investigators that the greater secretion
of gastric juice is counterbalanced by the retarding effect;
and hence, as a rule, there is no reason for or against using
PERSONAL HYGIENE 245
small amounts with meals, so far as the effect on gastric
digestion is concerned. But we shall see later (§§ 221, 223)
that alcohol affects other organs so harmfully that one
should decide against its use as a beverage.
219. Effect of Alcohol on Blood-System. — Small quanti-
ties of whisky or other strong alcoholic drinks, as used by
some physicians in cases of great depression of the heart,
stimulate that organ reflexly through the nervous system.
In large and intoxicating quantities alcohol is a direct and
powerful depressant of the heart, weakening the beat, dis-
tending the cavities, and diminishing the pumping of blood.
Herein is the scientific reason for the advice not to use large
and depressing quantities of alcoholic fluids for snake-bites.
It is well known that alcohol in even small amounts causes
flushing of the face and a sense of heat over the skin. This is
due to the dilation of blood-vessels. Habitual use of alcohol
very commonly leads to permanent dilation of the facial
blood-vessels.
The combined effect of large quantities of alcohol on heart
and blood-vessels is to lower the pressure of the blood.
Whether these effects are decidedly harmful depends upon
conditions, especially of organs other than the blood-system.
It is a significant fact that most modern physicians are careful
in prescribing alcohol, even in moderate doses, when there
is need of stimulating effects upon the heart and blood-vessels ;
for the harmful effects on other organs may more than coun-
terbalance any possible good done to the organs of circula-
tion by alcohol used as a medicine.
220. Alcohol and Respiration. — For a short time after
drinking alcoholic fluids there is an increase in the rate of
breathing. This is probably due to an increased loss of
heat from the dilated blood-vessels of the skin. In other
words, the respiratory organs must work faster in order to
246 INTRODUCTION TO BIOLOGY
supply oxygen for the increased internal oxidation needed
to supply heat in place of that lost. Such a chain of events
leads many great physicians to believe that harm will come
from a dose of alcohol, when as in pneumonia there is weakened
heart and lungs already congested with blood. At any rate,
it is exceedingly doubtful whether in conditions of health
any useful purpose is served by increasing respiration by
means of alcohol ; and hence so far as effect on breathing is
concerned, healthy people will be safest by avoiding alcohol
in any form.
221. Alcohol, Nervous Organs, and Muscles. — The gen-
eral influence of large amounts of alcohol upon the nervous
and muscular systems is well known to all who have observed
the actions of drunken men.
Alcohol tends to lessen all mental activities. Careful
experiments have shown that even a pint of wine diminishes
acuteness of smell and touch and interferes with the power
of the eye to estimate measurements. Psychologists have
failed to prove that alcohol evet" increases the quantity and
vigor of mental operations; on the contrary, even small
doses tend to lessen reasoning power. Larger quantities
affect the power of attention, judgment, and reason, render
the senses less acute, and exert an anaesthetic action which
leads to the sleep characteristic of intoxication. Study of
many such conditions leads eminent experts on drugs to the
opinion that, even in moderate quantities, alcohol tends to
have a sedative or narcotic action on the brain. In connec-
tion with this statement, may be cited the undoubted fact
that alcohol regularly used during the day's work diminishes
the amount and quality of work done. This has been ex-
perimentally proved by tests with type-setters and others
whose work is of such a nature that it is easy to compute
both speed and accuracy. Those who command armies and
PERSONAL HYGIENE 247
large groups of men engaged in physical labor agree that the
use of alcohol during work decreases effectiveness. This is
probably because of the sedative action above mentioned.
It has not yet been possible for investigators to collect
and collate the facts as to the effect of small amounts of
alcoholic liquors upon large numbers of brain-workers (law-
yers, teachers, clergymen, business men, physicians), for
the reason that many will not or cannot answer questions
concerning their own experiences with alcoholic drinks.
One list of 892 brain-workers in the United States showed
167 total abstainers, 579 occasional drinkers, and 146 mod-
erate drinkers. A large number of the moderate drinkers
expressed the opinion that the use of alcoholic drinks gives
bad results as stimulants to mental work, and many also
stated that they used alcohol from habit j and with no expec-
tation of being enabled to do more or better mental work.
In spite of the fact that we cannot say definitely how
little alcohol will seriously interfere with the normal func-
tions of the nervous system, it is clear that the hrain-worker
acts most wisely hy avoiding alcohol. Moreover, it cannot be
repeated too often that all people will probably gain in
both physical and mental efficiency if they avoid alcohol
altogether, because the possible accumulative effects of the
frequent use of small doses of alcohol are still imknown to
science.
Alcohol in Muscular Work, — Just as alcohol tends to
depress the activities of various organs, so it acts on the
muscles. It does not increase the power for hard muscular
work, and in numerous observations by commanders of
soldiers and of other groups of men it has been found that
alcoholic drinks are opposed to the most efficient muscular
work. Men can do far harder and better work under the
stimulating influence of such harmless hot beverages as beef
248 INTRODUCTION TO BIOLOGY
tea, chocolate, malted milk, and plain milk than they can
do when depending upon alcohol for stimulation of tired
nerves and restoration of weary muscles.
222. Nutritive Value of Alcohol. — " Is alcohol a food? "
The answer depends upon what we imderstand by food.
It is not food in the sense that bread and meat are foods, for
it cannot support life. It lacks the nitrogen and necessary
mineral elements for growth and repair. But it may, in
very small quantities, take the place of foods used for fuel or
energy, for some is oxidized in the human body.
However, it is of little moment that alcohol has a very
slight food value, for against it there are serious objections
as follows : (1) Only small amounts of. alcohol can be used
as food ; and it is easy to overestimate the amount which is
safe. (2) There is a peculiar tendency to excessive and
habitual use. (3) Its action as a harmful drug is likely to
overbalance its slight value as food. (4) It is a very expen-
sive food as compared with carbohydrates and fats, which
can supply equivalent energy.
Such grave objections to alcohol in any liquors make it
necessary to regard its food value as of no practical impor-
tance. One of the best of advanced books on physiology well
summarizes the whole matter as follows : " Only in very
exceptional cases can alcohol have any practical importance
as a nutriment. It is especially in the case of acute diseases
accompanied by diminished digestive power that alcohol
seems to serve as a valuable nutriment." However, it is
only fair to say that many doctors never prescribe alcohol.
After all, no one regularly uses alcohol as food, but rather
for its peculiar taste and stimulating effect. The use or
disuse of alcohol must depend upon the answer to the ques-
tion, "Is the stimulating effect of alcohol injurious?"
Except in small quantities it certainly is an injurious stimu"
PERSONAL HYGIENE 249
lant; and no one can safely estimate the quantity which may not
lead to accumulated effectSy or to habits of excess,
223. Disease Effects of Alcohol. — Many organs are
known to become diseased as the result of long-continued
excessive use of alcoholic drinks. Liver, kidneys, heart,
blood-vessels, and nervous organs are frequently involved
in disease changes. All medical men recognize that alcoholic
intemperance leads to an immense amount of sickness. A
large number of deaths are due to the diseases known as
chronic alcoholism and delirium tremens (a kind of tem-
porary insanity) ; and many cases of Bright's disease of the
kidneys, paralysis, pneumonia, tuberculosis, and many
other diseases are believed by eminent physicians to have
been hastened to a fatal issue by the previous use of alcohol.
It should be noted that no reputable physician claims that
even excessive use of alcohol will always lead to diseased
conditions. There are exceptional individuals who are
ahnost constantly intoxicated, and yet show no external
evidences of diseased organs. However, no sane person who
has learned of the great liability of excessive drinkers to dis-
eases will care to take his chances of being one of the few
who appear to escape the most serious consequences. More-
over, it should be noted that in recent years there have been
found many cases of diseases in " moderate drinkers " which
are probably due to long use of alcoholic liquors. These
are the reasons why the great life-insurance companies favor
total abstainers.
It is well known that the excessive use of alcohol leads to
obesity or storing of fat. This is most dangerous when in
the muscles of the heart. Gout is often, not always, caused
by alcoholic liquors.
224. " Pure '* Alcoholic Beverages. — Manufacturers of
alcoholic liquors often advertise that their products have
248 INTRODUCTION TO BIOLOGY
tea, chocolate, malted milk, and plain milk than they can
do when depending upon alcohol for stimulation of tired
nerves and restoration of weary muscles.
222. Nutritive Value of Alcohol. — " Is alcohol a food? "
The answer depends upon what we understand by food.
It is not food in the sense that bread and meat are foods, for
it cannot support life. It lacks the nitrogen and necessary
mineral elements for growth and repair. But it may, in
very small quantities, take the place of foods used for fuel or
energy, for some is oxidized in the human body.
However, it is of little moment that alcohol has a very
slight food value, for against it there are serious objections
as follows : (1) Only small amounts of. alcohol can be used
as food ; and it is easy to overestimate the amount which is
safe. (2) There is a peculiar tendency to excessive and
habitual use. (3) Its action as a harmful drug is likely to
overbalance its slight value as food. (4) It is a very expen-
sive food as compared with carbohydrates and fats, which
can supply equivalent energy.
Such grave objections to alcohol in any liquors make it
necessary to regard its food value as of no practical impor-
tance. One of the best of advanced books on physiology well
summarizes the whole matter as follows : " Only in very
exceptional cases can alcohol have any practical importance
as a nutriment. It is especially in the case of acute diseases
accompanied by diminished digestive power that alcohol
seems to serve as a valuable nutriment." However, it is
only fair to say that many doctors never prescribe alcohol.
After all, no one regularly uses alcohol as food, but rather
for its peculiar taste and stimulating effect. The use or
disuse of alcohol must depend upon the answer to the ques-
tion, "Is the stimulating effect of alcohol injurious?"
Except in small quantities it certainly is an injurious stimvr
PERSONAL HYGIENE 249
lant; and no one can safely estimate the quantity which may not
lead to accumulated effects^ or to habits of excess,
223. Disease Effects of Alcohol. — Many organs are
known to become diseased as the result of long-continued
excessive use of alcoholic drinks. Liver, kidneys, heart,
blood-vessels, and nervous organs are frequently involved
in disease changes. All medical men recognize that alcoholic
intemperance leads to an immense amount of sickness, A
large number of deaths are due to the diseases known as
chronic alcoholism and delirium tremens (a kind of tem-
porary insanity) ; and many cases of Bright's disease of the
kidneys, paralysis, pneumonia, tuberculosis, and many
other diseases are believed by eminent physicians to have
been hastened to a fatal issue by the previous use of alcohol.
It should be noted that no reputable physician claims that
even excessive use of alcohol will always lead to diseased
conditions. There are exceptional individuals who are
almost constantly intoxicated, and yet show no external
evidences of diseased organs. However, no sane person who
has learned of the great liability of excessive drinkers to dis-
eases will care to take his chances of being one of the few
who appear to escape the most serious consequences. More-
over, it should be noted that in recent years there have been
found many cases of diseases in " moderate drinkers " which
are probably due to long use of alcoholic liquors. These
are the reasons why the great life-insurance companies favor
total abstainers.
It is well known that the excessive use of alcohol leads to
obesity or storing of fat. This is most dangerous when in
the muscles of the heart. Gout is often, not always, caused
by alcoholic liquors.
224. " Pure " Alcoholic Beverages. — Manufacturers of
alcoholic liquors often advertise that their products have
248 INTRODUCTION TO BIOLOGY
tea, chocolate, malted milk, and plain milk than they can
do when depending upon alcohol for stimulation of tired
nerves and restoration of weary muscles.
222. Nutritive Value of Alcohol. — " Is alcohol a food? "
The answer depends upon what we understand by food.
It is not food in the sense that bread and meat are foods, for
it cannot support life. It lacks the nitrogen and necessary
mineral elements for growth and repair. But it may, in
very small quantities, take the place of foods used for fuel or
energy, for some is oxidized in the human body.
However, it is of little moment that alcohol has a very
slight food value, for against it there are serious objections
as follows : (1) Only small amounts of. alcohol can be used
as food ; and it is easy to overestimate the amount which is
safe. (2) There is a peculiar tendency to excessive and
habitual use. (3) Its action as a harmful drug is likely to
overbalance its slight value as food. (4) It is a very expen-
sive food as compared with carbohydrates and fats, which
can supply equivalent energy.
Such grave objections to alcohol in any liquors make it
necessary to regard its food value as of no practical impor-
tance. One of the best of advanced books on physiology well
summarizes the whole matter as follows : " Only in very
exceptional cases can alcohol have any practical importance
as a nutriment. It is especially in the case of acute diseases
accompanied by diminished digestive power that alcohol
seems to serve as a valuable nutriment." However, it is
only fair to say that many doctors never prescribe alcohol.
After all, no one regularly uses alcohol as food, but rather
for its peculiar taste and stimulating effect. The use or
disuse of alcohol must depend upon the answer to the ques-
tion, "Is the stimulating effect of alcohol injurious?"
Except in smaU quantities it certainly is an injurious stimv^
PERSONAL HYGIENE 249
lant; and no one can safely estimate the quantity which may not
lead to accumulated effectSy or to habits of excess,
223. Disease Effects of Alcohol. — Many organs are
known to become diseased as the result of long-continued
excessive use of alcoholic drinks. Liver, kidneys, heart,
blood-vessels, and nervous organs are frequently involved
in disease changes. All medical m^n recognize that alcoholic
intemperance leads to an immense amount of sickness, A
large number of deaths are due to the diseases known as
chronic alcoholism and deUrium tremens (a kind of tem-
porary insanity) ; and many cases of Bright's disease of the
kidneys, paralysis, pneumonia, tuberculosis, and many
other diseases are believed by eminent physicians to have
been hastened to a fatal issue by the previous use of alcohol.
It should be noted that no reputable physician claims that
even excessive use of alcohol will always lead to diseased
conditions. There are exceptional individuals who are
ahnost constantly intoxicated, and yet show no external
evidences of diseased organs. However, no sane person who
has learned of the great liability of excessive drinkers to dis-
eases will care to take his chances of being one of the few
who appear to escape the most serious consequences. More-
over, it should be noted that in recent years there have been
found many cases of diseases in " moderate drinkers " which
are probably due to long use of alcoholic Uquors. These
are the reasons why the great life-insurance companies favor
total abstainers.
It is well known that the excessive use of alcohol leads to
obesity or storing of fat. This is most dangerous when in
the muscles of the heart. Gout is often, not always, caused
by alcoholic liquors.
224. " Pure " Alcoholic Beverages. — Manufacturers of
alcoholic liquors often advertise that their products have
250 INTRODUCTION TO BIOLOGY
been " purified " so that they have no harmful action.
This is an absolutely false claim, for scientific studies have
shown that while the so-called " impurities '' in alcoholic
drinks are poisonous, they are present in such small amounts
that their efifect is slight, and that the harmful effect of
alcoholic liquors is chiefly due to the alcohol they contain.
Even absinthe and other highly flavored French liqueurs,
containing extracts of wormwood, anise, and other aromatic
herbs, certainly owe most of their decidedly harmful action
to their large amount of alcohol (50 to 80 per cent). Alcohol
in large quantities has been demonstrated to be poisonous
enough to account for most of the physiological evils
ascribed to alcoholic drinks. So long as people will drink
alcoholic fluids, there should be " pure food " laws aimed at
making them as free from poisonous substances as possible ;
but no one should be deceived by claims that a given brand
of liquor is harmless. Whenever alcohol is present in con-
siderable amount, there is a substance which, in a quantity
varying with individuals and conditions, is certain to be
harmful in its effect upon the essential life-processes.
The common opinion that the cheap artificial whisky sold
in some saloons for three cents a glass is especially injurious
to health as compared with '' high grade " natural whisky
made from corn and rye is not supported by chemical analy-
sis. This cheap whisky consists of 30 to 50 per cent alcohol
with caramel, sugar, and flavoring essences. Its harmfulness
depends chiefly upon the alcohol contained. Perhaps the
chief reason why such cheap liquors appear harmful is that
the low price leads to the use of two or three times more
alcohol. Certain it is, however, that the chief danger in
cheap whisky, and in all whisky j lies in the 30 to 50 per cent
of alcohol which it contains.
226. Alcoholic " Temperance Drinks.'' — Very many
PERSONAL BYGIENE 251
people who hold strictly to temperance principles are un-
aware that many fluids sold at drug-stores contain large
amounts of alcohol. In general, all ** tonics," " bitters,"
" malt-extracts," *' celery compounds," and other similar
fluids advertised as givers of strength, vigor, etc., contain
alcohol. Certain much-advertised medicines called " sarsa-
parilla " contain at least 25 per cent of alcohol. Many
" bitters " and " tonics " contain from 15 to 45 per cent of
alcohol. Some of these were formerly advertised as contain-
ing no alcoholj or as *' temperance " medicines ; but the
present drug law compels manufacturers to print on the label
the amount of alcohol and other drugs which may be very
injurious.
Root-beers, ginger ale, and fermented milk contain very
small amounts of alcohol, usually less than one per cent.
So-called '* sweet cider " sold by all dealers may have more
alcohol than the average beer, and frequently contains 0.2
to 3.5 per cent. " Hard " or fermented cider contains 4 to 8
per cent of alcohol, and therefore compares with mild wines
and strong beer.
226. Alcohol as Medicine. — Concerning the value of
alcohol in treating diseases, great authorities on medicine do
not agree. There are many eminent physicians who never
prescribe it, but prefer to use drugs whose stimulating action
is more definite and certain than that of alcohol. Other
equally eminent doctors hold that alcohol is of great value
in certain acute diseases where there is a tendency toward
general and heart weakness. On the whole, there is a
marked tendency towards dropping alcohol from the list of
valuable drugs.
227. Alcohol and Growth. — It is universally admitted
by physiologists that all alcoholic drinks are deleterious to
growing individuals, and this means the first eighteen or
252 INTRODUCTION TO BIOLOGY
twenty years of life. It is absolutely pernicious to young
children. And in addition to its direct physiological injury
to young people, there is the oft-demonstrated tendency to
excessive use. The vast majority of habitual drinkers of
alcoholic liquors begin in early life.
228. Summary of Effects of Alcohol. — Professor Atwater,
the famous chemist, who contributed much to our knowl-
edge of foods and their uses, wrote that in his personal
opinion " people in health, and especially young people,
act most wisely in abstaining from alcoholic beverages."
A conmiittee of five prominent American physiologists
has thus summarized the facts regarding the use of alcoholic
drinks : (1) They are not needed by young and healthy per-
sons, and are dangerous to them in so far as they tend to
create a habit. (2) In certain cases of disease and weakness
they are useful in quantities to be prescribed by physicians.
(3) Alcoholic drinks of all kinds are worse than useless to
prevent fatigue or the effects of cold, although they may at
rare times be useful as restoratives. (4) They are almost
always a useless expense. (5) Their use in excess is the
cause of much disease, suffering, and poverty, and of many
crimes.
229. Effects of Tobacco. — Concerning the effect of to-
bacco in its various forms upon health, there has been
much discussion. As is. well known, the stem and leaves
of the tobacco plant contain a poisonous substance known
as nicotine, which, in concentrated doses, quickly kills small
animals. However, this proves nothing regarding the effect
of smoking tobacco, or of the disgusting habit of chewing it,
which is now almost unknown among the better classes of
people ; for in both of these ways of using tobacco the nico-
tine is exceedingly diluted, as is the poison found in tea and
coffee. The result is that the effects of tobacco are not
PERSONAL HYGIENE 253
marked, and so even physicians are not always certain as to
its influence upon their patients. The best established
knowledge we now have is that indigestion, irritation of the
respiratory organs, and heart and nervous disturbances
may in some people result from the use of tobacco, while
others show no apparent eflFect.
All this refers to healthy adult men, for all medical authori-
ties agree that tobacco is always harmful to growing boys,
and interferes with their physical and mental development.
All schoolboys know how rigidly most athletic trainers for-
bid the use of tobacco by those in training.
The direct eflFect of tobacco is narcotic, and many smokers
say that it " soothes their nerves." It is very doubtful
whether the nerves would need " soothing " if the tobacco
habit had not been established. Opium also has such an
eflFect ; but it is well known that the craving for the soothing
is the result of the established habit, and those who never
used opium do not need to be soothed by that drug. Like-
wise, those who have never acquired the tobacco habit appear
to have no need for its narcotic or soothing eflFect. The
diflFerence between the opium and tobacco habits is in degree,
not in kind. Both create a demand or craving for their
peculiar narcotic eflfects.
The eyes of a few people are seriously aflfected by even
small amounts of tobacco, while many find that tobacco
smoke irritates the eye-membranes and causes some blurring
of vision.
The whole physiological truth about tobacco so far as now
known is that : (1) no one needs it except to satisfy an estab-
lished habit ; (2) many adults are injured by it, and no one
knows just how much will do harm to a particular person ;
(3) some adults are apparently not harmed by limited use ;
(4) it is decidedly injurious to growing boys ; (5) those who
254 INTRODUCTION TO BIOLOGY
avoid establishing the habit in youth do not as a rule care to
learn later, for there are no physiological reasons why any one
should deliberately set out to learn the use of tobacco in any
form.
230. Effects of Tea, Coffee, and Cocoa. — The first two
are most important because they are so widely used as
beverages. It is now well known to physicians that many
people drink too much tea and coffee, and that temperance
is needed in use of these {leverages no less than with alcoholic
drinks. Their stimulating effect is due to the presence of a
powerful drug (caffein), which has a stimulating action on
the nervous system. Nervousness, insomnia, headache, and
indigestion are common symptoms arising from excessive
use of tea or coffee ; and disturbances of other organs may
follow. One who uses even small amounts of tea and coffee
and who does not " feel well " for no apparent reason, should
try the effect of completely abstaining for a few days occa-
sionally. By so experimenting with themselves, many people
have learned that tea and coffee harm them. Other people
are certainly benefited by a limited use of these beverages.
Tea and coffee should never" be given to young children.
They may be harmed by such stimulants.
Cocoa and chocolate are made from the seeds of the cacao-
or chocolate-tree, a native of tropical America. The seeds
are rich with 50 per cent of fat, some of which is extracted
in the process of making commercial cocoa or chocolate.
Both of these contain a substance similar to the caffein of
tea and coffee, but much milder in its effect. One who has
difficulty from the use of tea and coffee will do well to experi-
ment with himself and thus learn whether chocolate or
cocoa produces marked effects.
231. Effect of Narcotic Drugs. — We are here concerned
with the effect of such drugs as opium, morphine, cocaine,
PERSONAL HYGIENE 255
laudanum, chloroform, chloral hydrate, and various patent
or secret preparations, all of which are habitually used by
some people. In most cases, such drugs are used first to
narcotize the nerves and thus relieve pain, and their frequent
use finally becomes a habit even more powerful than the
alcoholic habit. It is unnecessary to go into an extensive
discussion of narcotic drugs, for most intelligent people now
understand that only on a physician's advice is it safe to use
any drug to relieve pain ; and also that no drug should be
used so frequently as to oflFer the grave risk of establishing a
habit.
It should be noted that the narcotic drugs do not cure
diseases. For example, patent headache powders and pills
simply dull the sensory nerves. The headache may be due
to a disordered stomach, eye-strain, constipation, or other
causes ; and hence the narcotics give only temporary relief.
Obviously, it is wiser to consult a doctor, who may be able
to find the cause of the disorder and prescribe treatment.
The remarks made above concerning narcotic drugs might
well be applied to habitual taking of any kind of medicine
without a physician's advice. An inmiense amount of harm
is done by the thousands of patent medicines. Very few
of them are useful medicines, and all are sold at exorbitant
prices.
CHAPTER X
ORGANISMS THAT AFFECT HUMAN HEALTH
I. PLANTS THAT AFFECT HEALTH
232. Seed-Plants. — It is well known that many of the
seed-plants are poisonous. Some of these, such as the three-
leaf ivy, poison oak, and swamp sumac, produce in their
leaves and stems irritating substances which often cause
serious inflammations of the human skin. Many other
conmion seed-plants are poisonous when taken into the ali-
mentary organs, because they contain substances which are
absorbed into the blood. These substances are often ex-
tracted and in small quantities used in medicine, e.g.j strych-
nine, from a Java plant, aconite from monk's-hood, atropin
or belladona from the deadly nightshade.
Laurel leaves and wilted wild cherry leaves are known to
be poisonous to some farm animals. Several pamphlets
published by the United States Department of Agriculture
deal with poisonous plants.
233. Mushrooms. — The wild species of mushrooms and
toadstools are not commonly used as human food because
of the difficulty of distinguishing with certainty between
edible and poisonous species. There is no safe general rule ;
and botanists who have specially studied mushrooms advise
that no species should be eaten unless cautiously identified.
Especially should people avoid mushrooms with a ring or
cup at the base, for this is one mark of the deadly Amanita,
one of the most poisonous plants known to science. Atkin-
256
ORGANISMS AND HEALTH 257
son's " Mushrooms " and MarshalPs " Mushroom Book/'
and the pamphlets "Mushroom Poisoning" and "Edible and
Poisonous Mushrooms " (both issued by the United States
Department of Agriculture) give pictures and descriptions of
many species pronounced edible. But even with these
guides to identification one should.be exceedingly cautious
before deciding to eat a mushroom which resembles those
said to be poisonous.
(D) Exhibit pictures and speoimens of any available edible and
poisonous mushrooms. Especially should attention be called to
the Amanita. Mushrooms are easily preserved in 3 to 5 per cent
formalin in water.
234. Molds. — The common molds (§ 272) which grow
on various foods are not known to be harmful if small quanti-
ties happen to be eaten; but the discovery that moldy
meal and other foods cause illness of chickens, cows, and
other animals suggests that moldy or musty foods should
always be rejected as unfit for human use. Some physicians
believe that pellagra, a disease which is common in oiu*
southern states, is caused by molds in cereal grains used as
food; but the exact facts are not yet known. Some skin
diseases (e.gf., ringworm) are due to mold-like plants grow-
ing just beneath the surface of the skin. The spores of such
plants are commonly distributed by towels, sponges, combs,
and barber's tools. These articles are easily sterilized by
means of soap and hot water.
Laboratory work on molds is given in § 272.
235. Algae. — Simple plants belonging to the group of the
AlgsB (§ 283) sometimes grow very abundantly in reservoirs
and tanks used for storing water, and impart a very disagree-
able flavor and odor to the water. In some of the late
summer months of recent years the great Croton Lake that
s
258 INTBODUCTlcm TO BIOLOGY
supplies water to New York City has been polluted by these
plants. They produce no directly poisonous substance in
the water ; but affect health as do many other disagreeable
substances that influence appetite and digestion.
236. Microscopic Plants. — Man has long since learned
most of the larger poiso.nous plants which may affect health ;
but only within the past half-century has it been discovered
that some of the microscopic plants are the causes of the
most destructive diseases. These simple plants belong
chiefly to a group known as the hacteria, and the science
devoted to them is bacteriology.
Bacteria
237. Bacteria, Germs, Micro-organisms, Microbes. —
The word bacteria (singular, bacterium) is the biological
name of a group of the simplest one-celled plants. Popu-
larly, the four terms which stand in the above heading are
used as if synonymous ; but the fact is that the last three
may properly be applied to some microscopic organisms
which are not bacteria, and in certain cases are one-celled
animals (§ 258). In other words, the last three are general
words meaning microscopic organisms ; while bacteria are a
special kind of such small organisms, belonging to the plant
kingdom. Sometimes it is very convenient to have the
general terms. For example, the malarial germ (an animal)
and the typhoid germ (a plant — bacterium) both cause dis-
ease. Both are micro-organisms.
238. Study of Bacteria. — (L or D) Clean, by washing with
soap or soap-powder and rinsing, some small bottles or test-tubes
and plug with cotton. Place the plugged tubes in a sterilizer (§ 245
in " Applied Biology ") and keep the water boiling for a half hour.
Protect the tubes from dust until needed.*
* To Teachers : Concerning plugging tubes and sterilizing, see the "Ap-
plied Biology," § 245.
ORGANISMS AND HEALTH 259
Any clear soup or bouillon will serve for the following experiment.
The clear concentrated soups commonly sold at ten cents per can
may be used after diluting with water. Test with litmus-paper, and
if acid add some baking soda (or caustic soda solution), little by little,
until the soup is slightly alkaline in reaction. When thus prepared,
the bouillon may be kept in a bottle or flask by simply plugging the
mouth with cotton and sterilizing after each opening of the bottle.
BMll sterile test-tubes half full with the bouillon ; and replace the
cotton plugs, taking care that they do not become wet. Place the
tubes in the sterilizer for a half hour, and repeat on the following
day. The sterile tubes may now be kept indefinitely. It is best to
keep them covered so that dust from the air may not fall, on the
cotton and thus increase the chance of bacteria getting into the tube
when the plug is removed. *
Examine tubes which have remained sterile for many days. If
bacteria develop, the bouillon will become turbid or cloudy.
Take the plugs from some tubes with bouillon, and leave them
open for several days. Do bacteria develop ? What is the explana-
tion of the fact that plugged tubes remained sterile ? The cotton
certainly cannot keep air out of tubes ; what then can be its effect
on the air which enters the tubes ?
Mount a drop of bouillon from a tube which has become turbid.
It is well to have some cotton threads under the cover-glass to assist
in finding the focus. Examine with a high power. Look for ex-
ceedingly minute, transparent
bodies, much smaller than .••••••*!'* ca ^ .^ i"^
yeast cells and mold spores; ^ ^^^^^^^%^^fi H/f/ i*^ ^
some rod-shaped, some spheri- ^^ ISJ ^^<P^ ^^/v .«*.
cal, some twisted rods (Fig. ®^
71). By shifting the mirror of ^ ^J^^ ' - '
the microscope, it is possible "* — ^ —
to see them better. Some of Fig. 71. Various forms of bacteria, i,
these organisms {hacteria) micrococci^^ ^ bacilli with c^^^^^^
.J, , ^ . •!• chains of bacilli; 4. spirilla; 5, bactena
swim rapidly by means of ciUa ^^h spores; 6, dividing bacterium.
so small that they cannot be
seen with the ordinary microscope. Watch the swimming of some
of the largest bacteria visible. Remember that the apparent rate of
speed has been magnified as much as have the bacteria, and that the
distance apparently covered must be divided by the magnification of
the microscope used in order to learn the actual speed. Glass slides
260 INTRODUCTION TO BIOLOGY
with delicate ruled lines have been used to estimate the size and
speed of the bacteria. Some of those often seen are so small that
50,000 placed end to end would make a row one inch long, and a
bacterium Tffiirv of an inch long belongs to a giant species.'
. In order to make bacteria more distinctly visible, it is customary
to stain them. A simple method is described in § 255 of the !* Ap-
plied Biology."
Make brief examination of bacteria obtained from various sources,
such as sour milk, " mother-of-vinegar," and any organic materials
found in a state of decomposition. All bacteria seen will be spheri-
cal, rod-like, twisted, or spiral in shape, but they may be found
united in groups.
Bacteria may be found which show an enlargement due to the
formation of a spore. These spores appear as dark spots when the
microscope is slightly out of focus, and very glistening when in
focus. " Hay-tea," made by pouring hot water over some good
hay, and filtering through a layer of cheese-cloth, is excellent for
growing bacteria for study of spores. Fill several test-tubes half
full of " hay-tea," plug with cotton, heat to 100° C. by holding tubes
in gas-fiame until boiling occurs, or keep them for a short time in a
sterilizer. Sterilize half of the tubes again on second and third days.
The tubes sterilized once will probably become turbid within a few
days and microscopic examination will show that practically all the
bacteria present are rod-like. Later they will form spores; and
this is the secret of the bacteria appearing after once boiling. The
spores were able to withstand the heat ; but before the second or the
third heating they had germinated and the bacteria thus formed were
killed by the later heating.
Pure Cultures of Bacteria. Gelatin plates, (D) The teacher will
explain these two topics. Reference : " Applied Biology," § 255.
239. Important Facts concerning Structure and Functions
of Bacteria. — (a) Size, — The average rod-shaped bacteria
are not more than Trrirnr of ^^ i^^ch in length and Tnr^Tnr ^
diameter. Many of the spherical forms are not more than
sishru of ^^ ^^^^ i^ diameter ; some are still smaller and just
visible with the highest powers of the microscope, and the
germs (supposed to be bacteria) of the foot- and mouth-
ORGANISMS AND HEALTH 261
disease of cattle are so small that they will pass through the
pores of the earthenware tubes in a Pasteur-Berkefeld filter,
which removes all bacteria large enough to be seen with
the microscope. In fact, bacteria are so small that fractions
of inches are inconvenient measures; and biologists com-
monly state dimensions in micromillimeters or microns, one
of which is xi/inr of ^ millimeter, or about ^rW of ^^ iJ^ch.
A bacterium ^sooo i^^^h long would be 1 micron long.
(6) Growth and Division, — .Under favorable conditions, a
bacterium rapidly grows to full size and then divides into
equal halves. Each of these absorbs food, grows, and soon
divides again. Certain bacteria have been observed to
divide every twenty minutes. Some one has computed that
if the descendants of a single individual bacterium were to
keep on dividing once an hour for two days there would be
more than 280 millions. You can easily verify these figures
by applying the mathematical formulas for geometrical pro-
gression. At the rate of one division every half hour, there
would be 4772 billions in 72 hours, and the weight would
probably be more than 7000 tons. These figures simply
illustrate the enormous reproductive powers of the bacteria
and help us to understand how a few bacteria introduced
may soon produce enough to cause disease or the decompo-
sition of foods. But the truth is that the half-hour or hour
rate of division is never maintained for long at a time. Soon
food-supply is exhausted, their own excretions accumulate
and exert a poisonous influence, and such unfavorable condi-
tions tend to check the rapid rate of growth.
(c) Spore-formation occurs in some bacteria. Only a few
himian diseases, and those not very conmion, are caused by
bacteria which produce spores. This is a fortunate circum-
stance, for, as we have seen, spores are much more difficult to
kill than are the ^-ctive bacteria. As a rule, one bacterium
262 INTRODUCTION TO BIOLOGY
produces only one spore, and this germinates and forms one
bacterium. There is therefore no multiplication, and spore-
formation is simply a device for adapting the bacteria to
unfavorable conditions. Spores taken from cattle dead from
the anthrax disease have been found capable of germinating
many years after the animals were buried.
(d) Temperature, — Some bacteria are able to multiply
near the freezing point, and some live in hot springs in water
at a temperature which will kill most kinds of bacteria and
other living things. Between these two extremes there are
all gradations. Contrary to the popular belief, freezing
does not kill all bacteria. Bacteria of several species, in-
cluding those which cause typhoid and diphtheria, have been
kept several days at the temperature of liquid air (about
— 190° C), and when thawed out, have appeared to multiply
normally. However, a very large percentage of common bac-
teria die when frozen in ice, and comparatively few are living
after the ice has been kept five or six months. A few years
ago an epidemic of typhoid fever was traced to ice that had
been stored seven months, so that all ice from waters con-
taminated with sewage should be regarded with suspicion.
While most of the bacteria will probably die, the few which
remain may multiply rapidly when taken into the human
body.
The thermal death-point of bacteria varies. Ten minutes
exposure to a temperature of 70° C, or one minute at 100° C.
(boiling), will kill all bacteria of typhoid and tuberculosis.
Spores of other bacteria may survive the boiling point for
hours ; but boiling for a short time on successive days
(known as discontinuous sterilization) will cause the spores
to germinate, and then they are easily killed by heat. For-
tunately, the disease-producing (pathogenic) bacteria likely
to be in drinking water and milk do not form spores.
OBOANISMS AND HEALTH 263
Sterilizaiion of substances containing bacteria is usually
accomplished, as described for molds (§ 273), by discontin-
uous steaming or boiling on two or three days. The first
boiling (100° C.) swells the spores, and the later heating
kills them. Steam under about 30 lb. press\u*e and having
a temperature of about 130° C. will kill even resistant spores
in half an hour. Dry hot air at about 350° C. is also efficient
as a sterilizer of such articles as clothing.
Pasteurization (discovered by Pasteur) of milk means
heating it to a temperature of 60 to 70° C, and keeping at this
temperature for 10 to 20 minutes. It does not change milk
as does heating to boiling (100° C), but it kills the disease
bacteria likely to be present in impure milk.
(e) Ldght. — Strong sunlight kills all bacteria which are
directly exposed, as on the surface of soil. Some species are
killed in a few minutes, and few can withstand hours of ex-
posure. This is important, for it suggests the value of sun-
light for killing bacteria in rooms, and especially on cloth-
ing, carpets, etc., which can be exposed out-of-doors. It
also suggests the importance of building houses so as to get
the maximum of sunlight ; for example, shade trees should not
stand too near houses. The value of sunlight in disinfecting
streets cannot be too strongly emphasized. Sprinkling with
water increases the germicidal action, for wet bacteria stand
less light. It is imfortunate that all main streets in cities
cannot be in the north-south direction so that all their sur-
faces will get the full force of sunlight at mid-day.
(/) Chemicals. — Many chemicals kill bacteria, and are
called germicides or disinfectants. Some chemicals which
do not kill bacteria but prevent their multiplication are
called antiseptics. Corrosive sublimate (mercuric chloride)
and carbolic acid are two powerful germicides. Most strong
acids and alkalies are also germicides. Chloride of lime and
N
264 IKTBODUCTION TO BIOLOGY
washing powders are efficient germicides for general house-
hold cleaning. The proprietary disinfectants advertised in
the magazines are too expensive and some of them are
frauds. Sulphur gas from burning sulphur and formalde-
hyde are commonly used for disinfecting rooms. Formalin
is now used more than sulphur, because it does not bleach
colored articles and is not so poisonous to human lungs. It
is easily evaporated in a room either by a special lamp or
by the heat generated in slaking lime on which formalin is
quickly poured before closing the door of the room. A
still more effective method for fumigation consists in mixing
1 ounce of paraformaldehyde with 2 J ounces of potassium per-
manganate and then adding 3 ounces of water before sealing
the door of the room. This will disinfect a room of 1000 cubic
feet in four hours. Cultures of bacteria exposed in such
rooms are killed by the gas generated. Among common
antiseptics are very dilute solutions of formalin, carbolic
acid, boric acid, common salt, and many substances pro-
duced by plants (menthol, thymol, eucalyptol, camphor,
cloves, cinnamon, etc.). The preservative action on foods
of common salt, vinegar, creosote (on smoked meats), and
spices is due to the antiseptic power of these substances.
The more powerful preservatives, such as boric acid, sodium
benzoate, formalin, and salicylic acid, are sometimes used
in foods ; but there is danger of injuring the digestive organs
by amounts so small that the sense of taste does not guard
against them, as it does against excess of vinegar andconunon
salt. Hence, the food laws and reliable producers of canned
foods are opposed to the use of the above-named chemicals
which might be unhealthful in quantities that the consumer
could not detect.
240. Where Bacteria are Found. — They may be said to
be almost ubiquitous. They are abundant in soils; water
ORGANISMS AND HEALTH 266
of seas, rivers, and lakes ; in the bodies of animals and plants ;
in all dead organic matter in nature ; and in the air (except
on high mountains, in polar regions, and in uninhabited
desert areas of such immense size that living bacteria could
not be carried on dust from other regions). In fact, bacteria
are distributed wherever they can get food and proper tem-
perature for growth and multiplication. Unlike other organ-
isms, certain bacteria do not require oxygen from the air,
for they can get it by decomposing the organic substances
on which they live. Such bacteria are called anaerobic
(living without air). Some of these can live deep in the
soil ; but at depths below where roots of plants and earth-
worms penetrate, the soil is usually without bacteria. Hence,
water from deep wells is usually pure, unless there are open-
ings between strata which somewhere communicate with
lakes or other surface water.
The wide distribution of bacteria, especially on or in
everything connected with the inhabited surface of the
earth, makes it extremely difficult to eliminate them. In
fact, only in closed vessels, etc., in which all bacteria can be
killed by heat or chemicals, is it possible to keep any sub-
stance free from bacteria (i.e., sterile). This is the reason
for the careful work necessary in preserving many foods.
241. Diseases Caused by Bacteria. — We are so famiUar
with the idea that many diseases are caused by " germs "
or bacteria, that it seems scarcely possible that less than
thirty years ago no disease had been shown to result from
the growth of bacteria in the human body. It was in 1876
that Robert Koch, of Berlin, made pure cultures of rod-
shaped bacteria found in the blood of cattle and sheep
suffering from anthrax, or splenic fever, and gave the first
proof that bacteria cause disease. Six years later (1882)
Dr. Koch demonstrated beyond question that human tuber-
266 INTRODUCTION TO BIOLOGY
culosis is caused by a bacterium which he named Bacillus
tuberculosis; and in 1883 he proved that the dreaded dis-
ease, Asiatic cholera, is caused by another specific bacterium.
These discoveries aroused the interest of many investigators,
with the result that in less than thirty years there has been
built up the new science of bacteriology, which has already
been of inestimable benefit to the human race and bids fair
to lead soon to absolute control of some of the most danger-
ous diseases.
To-day the list of diseases known to be caused by bacteria
is a long one and each year becomes longer. The following
are some of the best known: tuberculosis of any organ,
cholera, diphtheria, typhoid fever, blood-poisoning, pneu-
monia, meningitis, influenza, tetanus, leprosy, bubonic
plague, and numerous others of rarer occurrence. These
are all infectious, or germ diseases ; and some of them are
also contagicms, or liable to be transmitted by contact with
patients.
242. Diseases Caused by other Micro-organisms. — A
number of diseases are now known to be caused by micro-
organisms which are animals. These animal parasites are
one-celled, and belong to the group of the Protozoa (§ 115).
The terrible African disease known as sleeping sickness,
malaria, and a form of dysentery are the best-known human
diseases certainly due to animals which produce effects similar
to those of bacteria in other diseases.
243. Infectious Diseases not yet Understood. — Other
diseases are almost certainly due to micro-organisms
which are readily transmitted from one person to another,
thus making them infectious ; but the organisms have not
been discovered. The diseases of unknown causation are
yellow fever, hydrophobia, smallpox, whooping-cough,
measles, scarlet fever, and mumps. The last five of these
ORGANISMS AND HEALTH 267
infectious diseases are highly contagious. In all these
diseases it has been possible for medical men to do much
towards controlling their spread by working on the assump-
tion that some undiscovered micro-organism is the cause,
and that therefore cases of these diseases should be handled
according to principles based on diseases known to be caused
by bacteria. For example, yellow fever has been shown
to be transmitted by bites of certain kinds of mosquitoes
which have previously sucked blood from a yellow-fever
patient ; and this has suggested the desirability of applying
the rules first worked out when it was discovered that malaria
is caused by microscopic animals which are injected into
hupian blood by mosquito bites. The rules simply require
destroying of the mosquitoes (§ 260), or preventing them
from biting healthy persons and especially those sick with
malaria or yellow fever. Similarly, in dealing with smallpox,
scarlet fever, mumps, measles, and whooping-cough, physi-
cians have assumed the existence of some undiscovered micro-
organisms, and have required isolation of patients, quaran-
tining of exposed persons, and disinfection of rooms and of
all articles on which bacteria from patients might have
lodged.
244. How Bacteria cause Disease. — We may use diph-
theria as an illustration of how bacteria cause disease.
We all know that the events connected with this dreaded
disease are as follows : A child is " exposed " to diphtheria,
perhaps in school where other pupils have shown signs of the
disease, and after a number of days the symptoms of the
disease may appear. Examination of the throat discloses
certain peculiar spots, and microscopic examination of ma-
terial from the surface of these spots shows thousands of
bacteria of the diphtheritic species. The reason why the
disease did not appear inmiediately after exposure is that
268 INTRODUCTION TO BIOLOGY
it required time for the one or few bacteria which first
lodged in the throat to multiply and form a colony.
Toxins. — The symptoms of diphtheria are not simply
those of a sore throat, for the patient is evidently afifected in
many other organs. This seems surprising, for the bacteria
are not so widely distributed ; but the explanation has been
found in the fact that the colony of bacteria excrete a poison
(toxin) which is absorbed by the blood and then distributed
widely in the body. Thus bacteria growing in one organ
(local infection) may profoundly affect other organs in the
body.
245. Antitoxins. — If the vitality of the patient is strong
enough to endure the diphtheria toxin for some days, the
climax of the disease is passed and convalescence ensues.
The explanation of this conquering of the disease is that the
cells of the patient's body have gradually secreted a sub-
stance which counteracts the toxin; this is an antitoxin.
Those patients whose bodies are not able to secrete enough
antitoxin succimib to the disease.
Every one who reads newspapers must have learned that
doctors now treat diphtheria with an antitoxin obtained
from horses. The explanation is as follows. A great many
children are not strong enough to make in their tissues anti-
toxin sufficient to overcome the poisons of the diphtheria
bacteria ; and it cannot be known at the beginning of the
disease whether the patient is going to be strong enough.
Hence, it is desirable to give some artificial aid. No drug
has been found to do this. But taking advantage of the fact
that diphtheria toxins will cause the horse tissues to form
antitoxin, which appears in the blood, physicians now inject
into the blood of the human patient some antitoxin from
horse blood, and this saves the child from being seriously ill
while his own cells develop antitoxin. Thus it is possible to
ORGANISMS AND HEALTH 269
put into the blood of a child on the first days of diphtheria
more antitoxin than the child might develop in his own cells
after days of illness.
Some health laboratories keep inoculated horses constantly,
so as to have a supply of antitoxin ready for use by doctors
who discover cases of diphtheria. The toxin injected into
the horses does not make them appear sick. The with-
drawing of comparatively small quantities of blood for ex-
tracting the antitoxin is done by an instrument which does
no serious injury ; and more antitoxin may be obtained from
the same horse every month.
246. Other Diseases are Similar. — This story of the rela-
tion of bacteria to diphtheria is very similar to that of many
other diseases now known to be caused by bacteria. The
bacteria enter the body, multiply and form toxins. Then
antitoxins or other opposing substances appear, counteract
the toxins, and the patient recovers. In only a few cases
has it been possible to find in other animals antitoxins which
can be injected into the human tissues to cure or prevent
human diseases.
Antitoxins and other antibacterial substances are specific.
For example, diphtheria antitoxin will not cure or prevent
any other disease, and the antitoxin which surgeons use after
injuries by Fourth-of-July pistols is obtained from animals
into whose bodies the toxins of tetanus or lockjaw bacteria
have been injected, causing the animal's tissues to make
tetanus antitoxin.
247. Immunity. — One of the most interesting things con-
nected with bacterial diseases is the fact that some people
never have certain diseases even when often exposed to infec-
tion ; and also it is a rule that one is not likely to have the
same diseases a second time. Furthermore, adults are not
likely to have the diseases which commonly affect ciiildren.
270 INTRODUCTION TO BIOLOGY
This lack of susceptibility to diseases is known as immunity.
Immunity which is present in human or animal individuals
who are not susceptible to a certain disease is known as
natural immunity, wh|le that which follows an attack of a
disease is said to be acquired.
Natural immunity is much more common than is suscep-
tibility to germ diseases. Probably the bacteria cause dis-
ease only in a small percentage of the individuals which
they enter. It is known that germs of pneumonia and other
diseases are often present in persons who show no signs of
disease. The reason why we do not develop disease every
lime a pathogenic organism enters our bodies is due to
(a) destruction of bacteria by the white cells of the blood
and lymph, (6) killing of bacteria by opposing soluble sub-
stances in the blood, (c) prevention of growth of bacteria by
antiseptic conditions in the body, and (d) counteracting of
toxins of the bacteria by antitoxins secreted by the cells of
our bodies. The relative value of these four methods of
protection varies with health and with individuals. In
general, all four are most efficient when there is good health,
and hence hygienic hving by improving general health helps
the human body in opposing pathogenic organisms. This
is well illustrated in the case of tuberculosis, for the important
factor in its prevention and cure is building up the general
health.
Vaccination is an example of acquired immunity produced
without an attack of smallpox, but by substitution of a simi-
lar and harmless disease known as cow-pox. In fact, cow-
pox seems to be smallpox which has been weakened by devel-
oping in cows ; and so when vaccine matter is taken from the
pustules on cows and rubbed into a cut or scratch on the
human skin, the result is a mild development of cow-pox.
This causes the human tissues to produce some opposing
0B0ANI8MS AND HEALTH 271
substance which eflfectually prevents the disease smallpox
for many years, the length of immunity varying with indi-
viduals.
No scientific man questions that vaccination against
smallpox has been one of the means (isolation and disinfec-
tion are others) which has made smallpox one of the rarest
diseases; and that it should be practiced whenever, in a
particular locality, there are cases which may have spread
infection widely. For example, when a case appears in a
school, all teachers and pupils in that school should be imme-
diately protected by vaccination performed by competent
doctors.
Protective Inoculation. — Similar to vaccination is protec-
tively inoculating by giving weakened doses of toxins.
Pasteur discovered that if the bacteria of cattle anthrax, or
those of chicken cholera, be grown in pure cultures in test-
tubes, the toxins get weaker as the cultures grow older. In
other ways, also, it is possible to weaken toxins of bacteria.
Now, if weakened toxins are injected first, there is a mild
attack of the disease. Then a stronger toxin will produce
no more effect ; and using in succession in a series of days or
weeks, stronger and stronger toxins, the animal into which
they are injected finally becomes unable to take the disease
and is said to be protectively inoculated. Many recent
experiments on thousands of soldiers seem to prove that it
will be possible to inoculate against typhoid in a similar way.
The destructive cholera of pigs and the distemper which
annually kills thousands of dogs are being studied in an
attempt to find a successful way of protectively inoculating.
Hydrophobia. — The treatment for this disease is another
well-known example of protective inoculation. Persons
bitten by dogs believed to have rabies go to a Pasteur Insti-
tute, and receive frequent injections of weakened toxins
272 INTRODUCTION TO BIOLOGY
obtained from the spinal cords of rabbits which have died
with the disease. The result of these weakened doses, which
are increased in strength with each successive injection, is
that the person treated acquires immunity; that is, he
probably develops an antitoxin faster than the toxins intro-
duced by the saliva of the rabid dog can act. At present,
no other method of dealing with this terrible disease is known,
and hence it is important that protective inoculation be ap-
plied as soon as possible. But much conunon sense is needed
in dealing with dog bites. There is no foundation for the
idea that dogs are Uable to go '* mad ^' in " dog days."
Most dogs which bite or appear sick are not rabid. Hydro-
phobia is a rare disease. One who is bitten by a dog should
have the wound properly treated by a surgeon, for blood-
poisoning bacteria may get into any deep wound. If the
dog shows signs of illness, it should be kept in confinement
for some days; for if it is rabid, the disease will develop
rapidly. If the dog has been killed, its body should be
expressed, packed in ice, to the nearest Pasteur Institute or
health laboratory. Microscopic examination of the nervous
organs will show whether the dog had hydrophobia, and it
can be quickly determined whether protective inoculation
should be given to the person who was bitten.
248. Few Bacteria cause Disease. — The student should
not infer from the preceding description that all species of
bacteria affect human health. The fact is that most species
now known are not pathogenic (disease producing). Cer-
tain species actually protect our health because they decom-
pose organic matter, such as sewage, which may contain
dangerous typhoid or other bacteria. These and other
useful bacteria will be described in § 271,
ORGANISMS AND HEALTH 273
Bacteriology Applied to Human Health
249. Bacteriology and Health. — The principles of bac-
teriology which have been discovered within the past thirty
years are not only of interest in connection with the cause
and cure of many diseases, but are also of much greater
importance in that they are capable of being applied so as to
maintain health. The relation of bacteriology to hygiene
is a vast subject, and we can take time only for a few of the
most important points. We shall consider (1) how to avoid
the disease germs which are widespread, and (2) how to
prevent the wide distribution of disease germs.
260. Avoiding Disease Germs. — The methods of avoid-
ing the introduction of disease germs into one's body depend
upon the nature of the disease and the causative organism.
As a rule, the germs are introduced into the alimentary canal
with food and water, into the respiratory organs, into the
blood by insect bites, or into wounds.
261. Infection through Alimentary Canal. — Typhoid fever
and Asiatic cholera are good examples of intestinal diseases
caused by germs which are spread by excreta. Imperfect
sewerage and insects may lead to contamination of various
foods (milk, vegetables, fruits, raw oysters) and drinking
water. In places where these diseases are common the only
safety for the individual is in the use of cooked foods, which
should be served hot or at least guarded from contami-
nation by flies, dirty hands, and dust. Salads and other
vegetables not cooked should at least be carefully washed
in water which has been boiled.
If there is reason to doubt the purity of drinking water,
heat it to the boiling temperature and pour into sterile
closed containers (such as earthen jugs). Longer boiling
is unnecessary and undesirable. These rules will protect
274 INTRODUCTION TO BIOLOGY
individuals. How to check the spread of intestinal diseases
and thus protect the public is a problem of sewerage, and of
insect control (see § 261).
Intestinal diseases of children, especially in warm weather
(" summer complaint ")> a,re largely due to stale milk. If
very clean, fresh milk is not available, then the milk should
be pasteurized (§ 239). Thoroughly wash and sterilize all
milk-bottles daily.
It should be remembered that some bacteria of decay
may develop in foods and cause unhealthful conditions.
This and the fact that some disease germs will multiply
in milk and other foods should lead to caution. Foods
showing signs of decomposition should, of course, be rejected ;
but more important is the protection of foods from dust, in-
sects, and growth of bacteria. Remembering that bacteria
do not floiu'ish when very cold, one thinks of the ice-box as
best for preventing growth; but rarely is an ice-box cold
enough to preserve highly decomposable foods (such as
soup, gelatin, milk) for many days. Best of all methods
is to heat to the boiling point daily the foods to be preserved
temporarily, and also to keep them in an ice-box from day to
day. Thus disease germs, if present, will be killed; and
ordinary decay, which might produce ptomaines (poisons in
foods), is prevented.
252. Infection through Respiratory Organs. — Tubercu-
losis is a good example. Tubercle bacilli may get into the
lungs (1) by close contact of healthy with tuberculous persons
(e.g., by kissing, and by using drinking cups or towels in
common) ; (2) from dust arising from the dried sputum of a
tuberculous victim; (3) possibly from milk of tuberculous
cows; (4) by flies which carry the bacteria from sputmn.
The first line of danger is easily avoided. Especially should
those who suffer from tuberculosis or whose friends are afflicted
ORGANISMS AND HEALTH 275
apply to the Society for the Prevention of Tuberculosis, in
New York City, for free circulars and follow carefully the
rules which they give for preventing the spread of the
disease. Doubtful milk should always be pasteurized, or
boiled. The danger from dust is impossible of avoidance by
individuals so long as the spitting nuisance continues. For-
tunately drying and sunlight kill most of the germs out of
doors, and good ventilation and cleaning reduce the danger
indoors.
So far as personal hygiene is concerned, the most impor-
tant preventive measure against pulmonary tuberculosis is
keeping in general good health by good food, outdoor exer-
cise, fresh air, good sleep, and avoiding colds. So long as the
body is in good condition there is little danger of the bacteria
getting a chance to flourish in the lungs; and even if the
disease has begun to develop, a cure may be effected by
careful attention to the rules of hygiene which physicians
and special books and pamphlets prescribe.
Many children's diseases (whooping cough, measles,
mumps, chicken-pox, etc.) are believed to be contracted
through the respiratory organs. The best protection is
(1) keeping away from cases of such diseases, and (2) avoid-
ing the use of pencils, toys, towels, handkerchiefs, drinking
cups, etc., which may have been in contact with the mouths
of other children, who may carry the germs although ap-
parently well. These and the biological rules given in § 255
will prevent most epidemics of children's diseases.
253. Infection by Insects. — • The relation of mosquitoes
and flies to disease is stated in §§ 260, 261. Constant
warfare should be waged against these insects.
254. Infection in Wounds. — Proper healing without in-
flammation depends upon keeping out bacteria. Wash
all cuts with weak antiseptics obtained from druggists (e.g.,
276 INTRODUCTION TO BIOLOGY
carbolic acid one part to 40 of water, or mercuric chloride
tablets in water as directed on bottle. Listerine or strong
alcohol for slight cuts). Deep cuts or punctures should be
dressed by a surgeon, for bacteria may have been pushed in
too deep to be reached without special instruments, and there
is danger of tetanus (lockjaw) or septicaemia (blood poison-
ing). Protect wounds with sterilized cotton and medicated
gauze. Adhesive tape should not be put on so closely as to
keep out the air; and collodion solution is of questionable
value for any except shallow and well-cleaned cuts. Always
consult a doctor at once if any marked inflammation develops
in any wound. Delay in such cases often proves serious.
256. Preventing Distribution of Disease Germs. — This
may be accomplished (1) by scientifically dealing with cases
of germ diseases, which we discuss below under " the bac-
teriology of sick-rooms " ; and (2) by public sanitary con-
trol of food-supplies, water, sewage, quarantines, and other
matters of public hygiene.
256. Bacteriology of Sick-Rooms. — Since it often hap-
pens, especially with children, that germ diseases must be
treated at home, it is important that every citizen should
understand the scientific principles which doctors prescribe.
All the rules depend upon the fact that micro-organisms are
numerous in the secretions and excretions of patients, and
must be destroyed by the methods of bacteriology.
(1) Isolation, — As soon as a person becomes ill, he should
remain in one room until the doctor is sure of the diagnosis.
If it proves to be a contagious disease, complete isolation
will be required by the health officers. Even in cases of
diseases not commonly isolated it is best that only the
nurse come into contact with the patient, for there is always
possible danger of spreading the germs to healthy people.
In the cases of all dangerous diseases, the doorway to the
ORGANISMS AND HEALTH 277
patient's room should be kept covered with a damp sheet to
prevent as far as possible the exit of dust particles.
(2) Disinfection, — Remember that all articles in contact
with a patient may bear the germs. To reduce the amount
of disinfection which will be necessary, remove all useless
carpets, curtains, and furniture as soon as the disease is
diagnosed; and thereafter remove nothing from the room
except under the strictest germicidal precautions. Hence
all clothing, bedding, handkerchiefs, etc., should be placed
in a tight receptacle, such as a wash-boiler, covered with
water, and boiled for twenty minutes before they can safely
be sent to a laundry or washed in tubs at home. The wash-
boiler should be kept in the patient's room, and not opened
until after boiling. Its outside should be wiped with a
cloth wet with strong washing-powder solution, or other
germicide, if necessary to take it to another room for boiling.
Spoons, dishes, tumblers, etc., should be placed in a bucket
of water (preferably hot and containing much washing-
powder) before removing from the patient's room, and then
boiled for 20 to 30 minutes. Food left by the patient should
also be boiled or burned. Playthings used by children
should be such as can be washed in germicide solutions,
boiled, or burned after convalescence. A doll or a " teddy
bear," if not burned, might be a very dangerous carrier of
germs. It is best to give the patient books ^or magazines
of little value, never those from libraries, and finally carry
them (in a closed bucket) to a stove or furnace.
The one who attends the patient should thoroughly wash
her hands frequently with hot water p^nd soap, or with germi-
cide solutions, before leaving the room. Neglect of this simple
precaution has often spread germ diseases. The habit
of expert nurses of wearing washable dresses should be
imitated as far as possible.
278 INTRODUCTION TO BIOLOGY
All excreta should be treated with strong chloride of lime
or other cheap disinfectants sold by druggists. Proprietary
disinfectants are expensive and rarely reliable.
No sweeping should be allowed in a sick-room. Dust
should be wiped up with a damp cloth which should after-
ward be cleaned in boiling water with washing-powder.
After recovery of the patient, fumigate (disinfect by fumes
or gas) the room with formaldehyde (§ 239, /), leaving in the
room all furniture, pictures, mattresses, books, etc., and
opening the drawers, wardrobe, etc., so as to allow free circu-
lation of the gas. Do not remove rugs, curtains, clothing,
or any article from the room until after thorough disin-
fection.
These precautions may seem extremely detailed, but
bacteriologists have shown that they are necessary in order
to safeguard against distribution of dangerous germs. It is
the duty of every citizen to aid in popularizing and putting
into practice this knowledge by which some of the most dan-
gerous diseases may be made extremely rare in occurrence.
257. Public Hygiene. — Since a large number of people
are ignorant of scientific principles or have no regard for
the health of other people, it has become necessary to insti-
tute public sanitary control of foods, water, sewage, quaran-
tines, etc., in all cities and to a certain extent in the country
at large. Thus the official representatives of the govejmment
are charged with the duty of protecting citizens against dis-
ease in cases where the individuals cannot exert control.
For example, a city board of health can force dealers to sell
only clean milk, but without such public control the indi-
vidual citizen must accept the impure milk which the dealer
offers for sale. Likewise, under public sanitary control,
meats and other foods can be inspected and made to meet
the requirements of the law; water can be obtained from
0BGANI8M8 AND HEALTH 279
the purest possible sources ; sewage systems can be arranged
to avoid danger from disease germs ; streets and other public
places kept clean; quarantine of dangerous diseases estab-
lished ; vaccination required ; spitting on walks prohibited ;
and still other measures in the interest of public health put
into legal operation. Such is the field of public hygiene
and the work of health officials, who aim to apply biological
principles and protect the citizens in numerous cases where
individuals cannot protect themselves. Every citizen should
get acquainted with the health laws in his locality and then
cooperate with the officials whose duty it is to get the laws
enforced.
II. ANIMALS THAT AFFECT HEALTH
We shall consider first some of the simplest animals (Pro-
tozoa) which cause diseases similar to those of bacterial
origin. Most important of these are the microscopic animal
parasites that cause malaria.
258. Malarial Organism. — Malaria in its severest forms
has long been one of the worst diseases affecting the human
race. Vast territories in some parts of the world have been
left practically undeveloped by civilized men because of
malaria. Little was known as to the cause of the disease
until after 1880, in which year a protozoan parasite was
discovered in red blood-cells. Before that time malaria was
commonly supposed to be caused by some poisonous vapor or
miasm which arose from swampy land. In fact, the very
word " malaria " means bad air. In 1897, Ross, an officer
of the British army, demonstrated that the malarial
parasites develop in the stomachs of certain mosquitoes
(§ 260) which have sucked blood from persons who have
malaria; and a year later other investigators showed that
mosquitoes which have obtained blood from a malarious
280 INTRODUCTION TO BIOLOGY
patient are able to transmit the parasites while sucking blood
from perfectly healthy persons. Many later studies have
made it absolutely certain that the Anopheles mosquito is
the carrier of the disease; and this is one reason for the
recent attempts at exterminating mosquitoes as far as pos-
sible (§ 260).
The effect of the malarial parasite upon the red blood-
corpuscles is as follows: Small bodies injected into human
blood by sucking mosquitoes attach themselves to blood-
cells and begin to burrow (they have amoeboid, amoeba-like,
movements). Soon there are formed in the blood-cell (§§ 6-24)
small bodies called spores. Then the blood-cell disintegrates
and frees the spores, which fasten themselves to new blood-
cells and repeat the development. Each time the spores are
freed by breaking of the blood-cells, the patient has the chill
and fever which are characteristic of the disease. It is well
known that malarial attacks commonly occur on alternate
days, that is, every forty-eight hours. This means that a
common form of malarial parasite requires two days for de-
velopment in blood-cells. Another variety of malarial para-
site takes three days for development into spores, and so the
chill of the patient comes at intervals of three days.
All these oft-repeated cycles of development in human
blood-cells are simply growth and cell-division, typical
asexual reproduction. When human blood from a malari-
ous patient is sucked into the stomach of a mosquito, pairs of
certain spores unite (a true case of fertilization) and the
zygote (combined cell) penetrates the stomach-wall of the
mosquito and becomes encysted. Sometimes there are
several hundred such cysts in the stomach of a single mos-
quito. Each of these encysted parasites eventually divides
into thousands of slender thread-like bodies (sporozoites),
which by way of the lymph-tubes get scattered in all parts
ORGANISMS AND HEALTH 281
of the mosquito's body, especially in the poison (salivary)
glands. It is these sporozoites which are carried along with
the poison when a mosquito plunges its beak into human
blood-vessels; and each sporozoite enters a red blood-cell,
where it divides into 6 to 24 spores.
The full development in a mosquito's body need not take
over ten days in warm weather.
When once started in human blood the repeated cycles
may go on for a long time, or spores remain which may long
afterward start a new attack of the disease. This can be
prevented by the use of quinine, which kills the parasites,
especially if taken when the fever is subsiding, which is
while the young spores are free in the blood-plasma. Killing
the parasites in human blood prevents infection of mosquitoes ;
and if all infected people could be treated with quinine system-
atically, it is probable that the disease could be stamped out.
Preventing the spread of the malarial parasite by preventing
the breeding of mosquitoes and by protection against bites
will be discussed in § 260.
259. Other Protozoan Diseases. — The terrible South
African disease known as the sleeping sickness, from which
more than half a million natives in the Congo region have
perished in the last ten years, is now known to be due to
a parasitic protozoan which is injected into hmnan blood-
plasma by the bite of a peculiar African fly. The parasite
swims freely in blood-plasma. Many experts on parasitic
diseases are now engaged in studying the sleeping sickness ;
but so far no satisfactory remedy has been discovered. It
is spreading rapidly, and has become one of the greatest
problems in Africa.
There are some suggestions that yellow fever, scarlet fever,
smallpox, and hydrophobia are caused by protozoan parasites
similar to those which cause malaria; but there is yet no
282 INTRODUCTION TO BIOLOGY
convincing proof. In the case of yellow fever, it has been
proved that a mosquito (not the species concerned with
malaria) transmits the unknown germ of the disease ; and so at
present the only known way of checking the spread of yellow
fever is to check the mosquitoes. This has been very suc-
cessful in Cuba apd in Panama.
Fortunately, most of thp protozoans which live in lakes,
rivers, and seas are unable to live in the bodies of animals
and humans, and hence are entirely harmless.
260. 'Mosquitoes and Diseases. — 'Careful investigations
made in recent years have proved that certain insects are
responsible for transmission of disease germs. The most
famous case is that of mosquitoes of the genus Anopheles
(Fig. 72), which inject the malarial organism (§ 258) into
the human blood-system. Numerous experiments have
made it certain that this mosquito is harmless unless
it sucks blood from a person who has malarial parasites,
and it is now certain that a bite from such an infected mos-
quito is the only way by which malaria can be acquired.
It is also quite certain that another species of mosquito
is responsible for transmission of yellow fever, the germ of
which is still unknown.
The discovery that mosquitoes are thus connected with
certain dreaded diseases has led to a study of the life-his-
tories and habits of these insects, with a view to reducing
the number of cases of malaria and yellow fever. The
following rules are now agreed upon by competent ento-
mologists.
(1) Multiplication of mosquitoes should be checked by
destroying their breeding places; for example, by draining
swamps, ponds, and other places containing stagnant water.
Even a rain-water barrel, open cistern or tank, buckets,
empty fruit-cans, cavities in trees and stones, — in short,
ORGANISMS AND HEALTH
Culex
283
FiQ. 72. Comparison of malarial mosquito and common culex. a, eggs;
6, position of larvaB at surface of water; c, position of resting adults;
d, wings; e, head appendages — a, antenna; p, palp; t, tongue. Compare
lengths of palps of the females. {From Jordan's * * Bacteriology,'* after other
authors.)
284 INTRODUCTION TO BIOLOGY
any place where a small quantity of water stands for a few
days may serve as a breeding place, producing thousands of
mosquitoes. As far as possible all such places should be
arranged for permanent drainage, and others, such as cis-
terns and barrels, should be tightly covered or screened with
netting so as to exclude mosquitoes which are about to lay
eggs.
(2) Mosquitoes should be destroyed in the larval stages
when it is impracticable to follow the rule above. Many
streams, lakes, etc., cannot be drained; but stocking them
with fishes will result in destruction of most of the mosquito
larvae. It is important in such cases that the banks of the
streams be freed from rubbish and graded so that there will
be no small depressions in which mosquitoes may lay their
eggs safe from the attacks of fishes.
Oftentimes temporary relief, pending permanent drain-
age, may be gained by spraying crude petroleum on the sur-
face of stagnant ponds. This will destroy the larvae when
they come to the surface to breathe. Oil cannot be used if
it is desired to preserve plants and fishes in the water.
(3) People should guard against infection by avoiding
mosquito bites, especially when in a region where malaria
is known to occur each year, or when there is an epidemic
of yellow fever in the southern states. The methods of
avoiding bites are very simple. Houses should be well
screened, and isolated mosquitoes resting on the walls and
ceilings of bed-rooms should be killed each evening. Persons
obliged to be outdoors at night should wear thick and loose
clothing, and mosquito-proof veiling around the head.
(4) Persons suffering from malaria should remember that
their duty to their fellowmen demands that every possible
precaution be taken against being bitten by mosquitoes. A
drop of blood from a malarious patient may infect a mos-
ORGANISMS AND HEALTH 285
quito so that later it may inject the malarial organisms into
healthy people. In cases of yellow fever it is now the rule
to quarantine the patient in a screened room, and then make
sure that no infected mosquitoes escape. This can be done
by closing the room tightly after removal of the patient
and burning sulphur, which will kill any mosquitoes concealed
in the room.
Especially should the above precautions be taken against
malaria when Anopheles mosquitoes are common. Hence,
it is important that this species be easily identified. (See
Fig. 72.)
261. Flies and Disease. — Probably more important than
mosquitoes as carriers of disease germs are the common
house-flies. It is a well-known fact that these flies persist in
walking on food, and long before disease germs were known,
careful housewives made strenuous efforts at keeping them
from kitchens and dining-rooms. Recent bacteriological
studies have disclosed some startling facts which should lead
to a general declaration of war against the house-flies. The
facts are these : A fly allowed to walk across a sterile gelatin
plate will leave in its tracks many bacteria previously ac-
quired by walking on filth. Now, if a fly walks on sewage
containing germs of typhoid or of other intestinal diseases,
or on sputum from a tuberculous patient, and later walks
on food or on dishes ready to be used for food or drinking
water, it may leave in its tracks dangerous bacteria, which
may be taken into the body with the food or water, and then
cause disease. It is obvious that in this way a single house-
fly may be a very dangerous animal.
It has recently been discovered that the biting stable-fly
may transmit the germs of infantile paralysis.
There are several ways of combating this dangerous pest :
(1) Manure piles and similar breeding places should be re-
286 INTRODUCTION TO BIOLOGY
moved. (2) Houses should be carefully screened and fly-
poisons, traps, etc., used to kill the few that succeed in
entering. (3) All foods should be carefully guarded against
flies. (4) Arrangements for sewage disposal should be such
that flies cannot distribute bacteria. For this reason sewers
and cesspools that discharge into porous drain-tiles below
the surface of soil are preferable for coimtry and village
homes.
262. Other Insects and Disease. — Mosquitoes and house-
flies are the most important insects connected with diseases
in America, but other insects may likewise affect human
health. The germ of the terrible African sleeping sickness
is injected into the blood by the bite of a peculiar fly. The
bacteria of bubonic plague is probably transmitted by fleas
which infest rats in whose blood the germs occur. In fact,
any biting insect which gets an opportunity to bite any
person or animal whose blood contains disease germs may
become a carrier of the germs ; and any insect which comes
in contact with germs and later with human food may in-
directly cause disease.
The above account suggests the general relations of cer-
tain insects to human diseases. For more facts on this very
important subject read the pamphlets on "Mosquitoes"
and " Insects and Health '' published by the United States
Department of Agriculture.
263. Tape-worms. — All the worms known as tape-
worms are parasites in the alimentary canal of vertebrate
animals and humans. They have no mouth or digestive
organs, but absorb through their skin the digested food in
the alimentary canal of the host in which they live.
A tape-worm's body resembles a long, narrow ribbon or
tape, but is unlike a ribbon in that it is divided into seg-
ments (Fig. 73, c), and is narrow near the anterior end. The
ORGANISMS AND HEALTH
287
head is a rounded knob, with a circle of hooks and four
suckers by means of which the worm holds fast to the lining
of the intestine. There are nerves and excretory tubes in
each segment. Each of the larger segments towards the
posterior end has a complete hermaphroditic reproductive
system, consisting of ovaries, spennaries, and their ducts
leading to the surface. The extreme posterior segments
become greatly distended with enormous quantities of fer-
tilized eggs, each inclosed in a hard shell; and one by one
these "ripe" segments drop off and are carried out of the
intestine with indigestible matters or feces. As segments
drop off the worm,
new ones are formed
by new grooves in
the segments back
of the head. Thus
the oldest segments
are the most pos-
terior ones.
At the time a
"ripe" segment is
discharged from
the intestine, each
egg-shell contains
a small embryo
with six hooks. If
these small em-
bryos, in the case
of the human tape-
worms, happen to
fall on grass or other
food of pigs and ^^* '^^' Tape-worm, a, head with hooks and
f+l 4-\\' A' +' suckers; b, bladder-worm stage; c, parts of an
cattle, tne aigestive adult worm, oldest segments are largest.
288 INTRODUCTION TO BIOLOGY
fluids in the stomachs of these animals dissolve the hard shell
and free the embryo. Then it bores into some organ of its
host, becomes encysted, and develops a larva which is bladder-
like with a tape-worm head (Fig. 73, 6). This stage (called a
bladder-worm or cysticercus) in the tissues is dangerous ; for
if uncooked pork or beef be eaten by man, an encysted bladder-
worm may attach itself to the wall of the human intestine
by means of its hooks and suckers and develop into a tape-
worm.
The above account of the human tape-worms, of which
one species develops its bladder-worm stage in pigs and an-
other in cattle, represents the life-history of all the tape-
worms which inhabit the intestines of various vertebrates.
Two hosts are required, one for the tape-worm, and one for
the intermediate bladder-worm stage. One species of tape-
worm of dogs has its bladder-worm in rabbits ; one of sea-
gulls has its in earth-worms ; and one of cats has its in liver
of rats and mice. In Oriental countries there is a large
hmnan tape-worm whose bladder-worm stage is passed in
certain fishes. The bladder-worm stage commonly occurs
in the particular animal which is a favorite food of the host
of the fully developed worm.
Tape-worms are troublesome parasites in that they inter-
fere with the nutrition of the animals which they inhabit.
They are difficult to remove, because the head is so firmly
attached to the lining of the intestine. All the segments
might be dislodged by powerful drugs, but if the head re-
mained, it might continue to grow new segments.
Preventing infection of humans by the larvae of tape-
worms is a simple matter; namely, eat no meat raw or
" cooked very rare." In the case of pork there is an ad-
ditional reason for this rule, in that the far more dangerous
parasite Trichina (§ 264) may be present. With increasing
ORGANISMS AND HEALTH 289
attention to sanitation and disposal of sewage by bacterial
methods, there will be less chance of tape-worm embryos
getting into pigs and cattle and then indirectly entering
human beings. The rarity of human tape-worms in United
States is probably in part due to the fact that our farms are
more sanitary, and being larger than those of Europe, farm
animals live farther away from human dwellings.
264. Round Worms. — A good example of a harmless
round worm is the vinegar-eel, which is often abundant in
unfiltered vinegar, and especially in " mother-of- vinegar.''
They should be examined with the low power of a microscope.
Many species of the round worms are parasitic in intestines
of man and animals. Some are slender threads half an inch
long, while others may be a foot long.
The most terrible round worm is the trichina. The adult
worms live in the intestine of man, pig, and Other mammals.
The males are about 1 mm. (^ inch)
and the females 3 mm. long. The eggs
develop inside the female, and the young
(1000 or more) are bom alive ; that is,
discharged as young worms. These pass
through the walls of the intestine of
their host, and reach such muscles as
those of the arms, legs, and back. Each
worm embeds itself in a muscle-fiber
(Fig. 74), and a cyst forms around it. p^^~ 747 TrJw
It may remain encysted for years. An encj^ted in muscle.
ounce of infected pork may have 80,000 ^^^^"^ LeuckarD
such cysts. If the flesh be eaten by another animal or by man,
the cysts dissolve in the digestive organs, the young worms
develop, form eggs and sperms, and fertilized eggs develop
into young worms which become encysted in muscles. In a
town in Germany, in 1884, the flesh of one pig infected 364
u
290 INTRODUCTION TO BIOLOGY
persons, and 57 died within a month. This large number of
infections is easily understood when we calculate from the
figures above. A single ounce of pork might introduce into
a human stomach 80,000 trichinae. Suppose one half of
these are females, each able to produce 1000 young worms,
and four million encysted worms might be the result. Thus,
at this ra:te, 100 pounds of pork might contain enough cysts
to develop 64 billions of encysted worms, or 160 million for
each of 400 persons who might eat the pork. And this is
not the whole story, for a female trichina may live in the
human intestine and frequently give birth to as many as
1000 young.
There is no way to destroy the worms when once they get
into the human intestine. If the infected individual does
not soon die from the inflanmaation caused by the encysting
in the muscles, the cysts soon become hardened and there is
no more danger to the patient. Prevention is very simple ;
namely, eat no pork which is not well cooked. In some
countries, government inspectors examine meats at the great
slaughter houses, and condemn as unfit for human food all
meat found to have trichinae. If the parasites are present in
a pig, they are likely to be so abundant that a small piece
of lean meat (muscle) examined with a microscope will re-
veal the trichinae. However, such inspection is not infal-
lible, for the parasites have been sometimes overlooked by
expert inspectors.
The above account has not explained how pigs get in-
fected. Since they do not eat human flesh, they must get
the parasites from some other animal. Trichinae are found
in rats, and it is well known that pigs will eat dead rats.
Also, pigs eat scraps of pig meat such as are thrown away
by butchers on farms.
265. Poisonous Arthropods. — The important groups of
ORGANISMS AND HEALTH
291
arthropods are the crustaceans (crabs, lobsters, shrimps),
the arachnids (spiders, scorpions, mites, ticks), the myrio-
pods (centipedes), and the insects. The crustaceans are all
harmless, except that some of the large forms may produce
painful wounds by their pincer-like claws. The scorpions,
foimd in warm countries, have a sting at
the end of the *' tail," which is really a
part of the abdomen. Some large ones,
six to eight inches long, can seriously
poison man, and the sting of even the
small ones is painful. The poison-glands
of spiders secrete a fluid which is injected
through the hollow biting jaws and may
kill small animals; but is not fatal to
larger animals or man. Possibly people
with weak hearts have died from fright,
but not from the direct efifects of the
poison. The harvestmen or daddy-long-
legs resemble long-legged spiders, and
are popularly, but wrongly, supposed to be poisonous to
humans. Centipedes have poison-glands whose secretion
is conducted to the hooked ends of the first pair of
feet. The large tropical species may kill small animals
and cause great pain to humans; but the common ones
of temperate climates cause no more pain than does a
mosquito bite. Among the insects there are many able to
produce painful injuries to human skin. The females of
the bees and wasps have stings at the end of the abdomen.
The hair-like spines on certain caterpillars carry a very
irritating fluid. The bite of the female black flies sometimes
produces painful inflammation. Mosquito bites are some-
times said to cause blood poisoning, but this results from the
introduction of bacteria, not from the irritating saliva of the
A B
Fig. 75. Two kinds
of myriopods. A,
milliped; B, centi-
pede. {From Thom-
son.)
292 INTRODUCTION TO BIOLOGY
insect. A blistering fluid is secreted by the blister-beetles,,
which are known in pharmacy as *' Spanish flies " or can-
tharides. The poison from a single insect cannot kill a hu-
man with good heart action, but the combined effect of many
stings is certainly dangerous. Many insects are tradition-
ally credited with being harmful in peculiar ways, such as
earwigs that were once supposed to enter people's ears
during sleep, dragon-flies that were said to use their long
" tails " (abdomens) as darning-needles to pierce children's
ears, and many other such stories equally absurd in these
days when science has given us an understanding of the
structure and life of insects.
266. Dangerous Vertebrates. — Among the fishes, the
sting-rays have caudal spines without poison, while the fin
spines of toad-fishes and certain cat-fishes have poison-glands.
Wounds from any of these are very painful and may be fatal
if blood-poisoning bacteria happen to be introduced. The
electric-rays or torpedoes, the electric-eel of Paraguay, and
the electric cat-fish of the Nile have electric organs able to
give severe shocks when the fishes are touched. The flesh
of most fishes contains no poisonous substances when fresh.
Ptomaine poisoning from fish meat is caused by decomposi-
tion by bacteria and the poisons are the same as might
develop in any other spoiled meat. A few tropical species
of fishes are said to have poisonous flesh.
None of the amphibians (frogs, toads, and salamanders)
is dangerous; and even the supposed production of warts
by toad secretion is zoological myth.
Poisonous snakes belong to many different families. The
American moccasins, rattlesnakes, and copperheads, and
the Old World cobra, adders, and vipers are the most poison-
ous reptiles. Official figures show that in India alone more
than 20,000 people die annually from snake bites, but there
ORGANISMS AND HEALTH 293
are few fatal cases in the United States. Pythons, boas,
and anacondas are snakes which kill animals by constricting,
but it is doubtful whether they have killed men and large
mammals. Most of the snakes of the United States are not
poisonous. Snake poisons are secreted by glands in the
upper lips, and the " poison fangs " are upper teeth with
grooves or tubes for conveying poison beneath the skin of
the victims.
The remedies for snake bites are briefly as follows : —
(1) Place tight ligature on arm or leg to prevent circula-
tion of blood; (2) cut and enlarge the punctures made by
fangs in order to drain away as much poisoned blood as
possible; (3) wash out cuts with wine-colored solution of
potassium permanganate ; (4) take very small doses of alco-
holic stimulants, enough to cause increased pulse-beats;
larger doses are dangerous; (5) consult a good surgeon as
soon as possible, because blood poisoning may result from
the wound, and also it may be necessary to have stimulants
and anti-venomous serum injected hypodermically ; (6) keep
the wound covered with a cloth or cotton wet with some
antiseptic solution.
No bird or manmaal has poison-glands, and the flesh of
all of them is edible if one considers the absence of poison
rather than palatability. It is true that bites and scratches
from the beaks, jaws, and feet of birds and mammals have
been known to cause blood poisoning and tetanus, but
introduced bacteria are known to produce the poisons.
Mammals, such as dogs and wolves, often cause hydro-
phobia (§ 247) by their bites, probably by transferring an
unknown micro-organism that produces a poison or toxin.
267. Summary of Organisms Affecting Health. — This
chapter has given a brief account of some of the most im-
portant organisms that are known to affect human health.
294 INTRODUCTION TO BIOLOGY
Many of these are rapidly coming under control of man.
With modern firearms we no longer fear the large and
dangerous manmials. We know how to dress wounds so as
to prevent the growth of blood-poisoning bacteria which
various animals may introduce with their bites and stings.
The poisonous snakes rarely injure those who take proper
precautions (such as wearing strong boots, which the natives
of India do not do for protection against the deadly cobra).
Many insects which carry the germs of disease are now
recognized, and we are learning how to destroy them or to
prevent their bites. The 'chief poisonous plants which we
must avoid eating or touching are well known. Science
has made wonderful progress in fifty years against the micro-
organisms that cause some of the most dangerous diseases.
A large part of the highly efficient science of public hygiene
or sanitation is concerned with preventing the distribution of
the dangerous micro-organisms ; and the science of medicine
is making rapid progress in preventing and curing some of
the germ diseases. In short, the world is a vastly safer place
than it was one hundred years ago, because biological science
has so extensively investigated the organisms that affect
human health.
CHAPTER XI
THE ECONOMIC RELATIONS OF ORGANISMS
268. Many plants and animals are of interest to us in
economic ways ; that is, they are either useful or harmful.
For example, horses and cows and corn are of economic
importance because we make use of them, while some insects
are of economic importance because they destroy useful
plants that have a pecuniary value. Some of the most
important of the economic relations of animals and plants
are those of cultivated plants and domesticated animals,
and hence of very direct interest to farmers. The economic
relations of these useful animals and plants grown on farms
also concern those of us who happen to be living in cities,
for everybody's supply of food and clothing depends upon
the industry of agriculture which supplies the necessary
animal and plant materials out of which our foods and
clothing are made. As an illustration, if insects destroy the
wheat crop in several States, or disease kills many thousands
of animals (e.g., hog cholera in 1911), the price of bread and
meat will rise in both country and city. Not only do the
farmers lose what they could sell for much money, but
every person's living expenses will be affected by the increased
prices due to decreased production. Hence, we are all
interested in good crops on the farms ; and it is well known
that our national prosperity is greatest when the farmers
succeed in producing a large supply of such animals and
plants as are needed especially for food and clothing. In
295
296 INTRODUCTION TO BIOLOGY
short, the economic problems of animals and plants are not
limited in interest to those who grow them for market ; and
many intelligent city people are much interested in the suc-
cess of the farmers, for they realize the dependence of every-
body upon agriculture. In fact, if the farmers do not learn
how to increase their production of foods by making the
land more fertile and their management of animals more
scientific, the time will come in America when there will
not be grown enough animals and plants to feed liberally
several hundred millions of people.
These illustrations of the relation between animals and
plants and our everyday life will help the reader of the
following pages to see how many of the organisms men-
tioned may be of economic interest to very many people.
Most of the economic relations selected for study afifect all
of us, in both country and city. In some cases there are
both hygienic and economic relations of great importance ;
for instance, if tuberculosis of cattle is neglected and al-
lowed to become widespread among dairy cows, it will not
only make milk unhealthful, but the ultimate destruction
of the diseased cows will make it necessary for the dairjmaan
to charge more for the milk in order to balance the financial
loss from cows destroyed. In such a case both the health
and the pocketbooks of all people who use milk may be
affected; and hence we recognize that the bacteria which
cause the tuberculosis of cattle are of economic importance
to city people as well as to farmers.
Having thus considered how economic problems of biology
are of importance to everybody, we may go on to study
some of the most important relations of animals and plants.
ECONOMIC BELATI0N8 OF ORGANISMS 297
I. PLANTS OP ECONOMIC IMPORTANCE
(a) Bacteria
269. Bacteria in Foods. — In the preceding chapter we
have considered the relation of bacteria to human health so
far as they are the direct causes of disease. It is possible
that bacteria might indirectly cause some disease, such as
disturbances of the digestive organs, because they decompose
foods ; and so the relation of bacteria to foods is partly a
matter of hygiene and might properly have been referred
to in the preceding chapter devoted to health problems.
However, most foods which are noticeably decomposed by
bacteria are thrown away, and hence there is economic loss
rather than harm to health. Vast quantities of milk, meats,
and canned goods are spoiled by bacteria before they reach
the consumers; while in the average home there is much
financial loss from the action of bacteria on prepared foods.
This loss is in part reduced by cold storage of foods in ice-
boxes ; by preserving with the aid of such antiseptics as
common salt, vinegar, spices, arid strong sugar sirup; by
sterilizing with heat ; and by sealing in preserving jars so
as to prevent entrance of bacteria. Ice-boxes are useful
only for temporary storage, for they commonly have a tem-
perature several degrees above freezing and certain bacteria
develop. Sterilizing by heat is the most valuable of all
methods for food preservation ; but for the greatest success
certain precautions must be observed : Boiling once does
not kill spores, and hence peas, beans, sweet corn, meats,
and soups must be boiled on three or four successive days,
or else subjected to steam pressure of 20 to 30 pounds, in
order to preserve them in cans or fruit-jars.
The reason why tomatoes, cherries, plums, and many
fruits are easily preserved if boiled once and packed in
298 INTRODUCTION TO BIOLOGY
sterilized cans while hot, is that their composition is not
favorable for growth of bacteria. In fact, they are most
liable to fermentation by yeast plants, and these are killed
by heating to boiling temperature. They also favor the
growth of molds whose spores do not readily germinate in
sealed cans.
270. Bacteria and Diseases of Domesticated Animals. —
A very important economic aspect of bacteria is their re-
sponsibility for many destructive diseases of domesticated
animals. In recent years it has been discovered that a
large number of cows and pigs are infected with tuberculosis,
which many bacteriologists think is the same as the human
form of the disease. But even if it were not probable that
the disease can be transmitted from cattle to humans, it is
of interest because it is now causing enormous financial
losses. The scientific agriculturists of this country are now
very much alarmed and are attempting to check the dis-
ease by means of quarantining, slaughter of diseased cattle,
disinfection of stables, and sterilization of milk fed to calves
and pigs. Many other diseases of cattle, horses, pigs, sheep,
poultry, and dogs are caused by bacteria. In general, bac-
terial diseases of animals are like the human infectious dis-
eases in that they are more easily prevented than cured.
Isolation of sick individuals, disinfection of living quarters,
pure food, pure water, and protective inoculation or vaccina-
tion are some of the essentials in preventing the widespread
distribution of destructive germs. Even with these precau-
tions, it sometimes happens that a disease of animals becomes
epidemic. As an example, several million dollars' worth
of hogs have been killed within the years 1910-1912 by hog
cholera, the germs of which have become widely scattered,
especially in the States north of the Ohio River. At present
(1912) thousands of hogs are being protectively inoculated
ECONOMIC UELATtONS OF ORGANISMS 299
with the hope of rendering them immune to the cholera
bacteria, which are now so abundant on many farms that
for several years there will be danger of a new (epidemic.
271. Useful Bacteria. — The popular impression is that
all bacteria are harmful in that they decompose our foods or
cause dangerous diseases of man or of useful animals. This
is far from true, for very many bacteria are directly useful.
(a) Bacteria in Soils. — Most important of the useful bac-
teria are those which deal with the nitrogen compounds in
the soil. The nitrogenous excretions of animals and the
dead bodies of animals and plants must be changed by
bacteria before the contained nitrogen is ready for use by
plants. Moreover, certain kinds of bacteria found in the
tubercles on roots of beans, clovers, and their allies are able
to collect nitrogen from the air and to use it in elaborating
nitrogenous plant food. For a fuller account of bacteria
in soils, see '' Applied Biology,'' § 258.
(6) Bacteria in Milk. — The most common bacteria in
milk are those which produce lactic acid, which causes milk
to sour and coagulate. While this process may be undesir-
able in milk intended for drinking, it is useful in the making
of certain kinds of cheese. Moreover, the presence of the
lactic bacteria in milk prevents the growth of certain species
which cause putrefaction. If milk be heated to 70° C.
(pasteurized), the lactic bacteria are killed and others capa-
ble of producing putrefaction remain.
(D) A bottle of raw milk and one of pasteurized, left covered
just as delivered by the milkman, should be placed side by side,
either in an ice-box or on a table in the schoolroom, and examined
as to appearance and odor after two or three days. It is also inter-
esting to add a little sour milk (i.e., with lactic bacteria) to a bottle
of freshly pasteurized milk, and note that the putrefactive odors
are prevented while souring occurs.
800 INTRODUCTION TO BIOLOGY
There is no reason to think that the lactic bacteria in milk
are harmful when taken into the digestive organs ; but most
people do not like to drink sour milk. In recent years, we
have had many newspaper articles on Professor Metch-
nikoff's method of preparing milk for preventing excessive
development of decomposing bacteria in the intestines.
Such bacteria produce poisonous substances which may be
absorbed by the blood and cause ill health (auto-intoxication,
meaning self-poisoning). The milk prepared by the Metch-
nikoff method is " soured " not by the ordinary lactic bac-
teria but by adding pure cultures of a certain species. Phy-
sicians are not yet agreed as to the medicinal value of such
prepared milk ; but one who wishes to try it should purchase
the milk in which the proper bacteria have been grown.
There are at drug-stores tablets which are said to be con-
centrated extracts of the proper bacteria, but they are not
reliable.
(c) Bacteria in Butter and Cheese. — The peculiar flavors
of butter and cheese are due to certain species of bacteria,
and hence the scientific dairyman gives special attention to
methods which will prevent the growth of undesirable species
and favor the desirable ones. By using pure cultures of bac-
teria (and sometimes certain molds), it is possible to produce
the desired quality of butter and cheese. The disagreeable
flavor of butter made in many farm-houses is due to un-
desirable bacteria.
(d) Bacteria in Vinegar, — The change of alcohol into
vinegar by bacteria has been mentioned in connection with
yeast. " Mother-of-vinegar '' is a mass of bacteria imbedded
in a gelatinous substance which they secrete.
(D) Small pieces of the " mother '' should be examined especially
with the microscope.
(e) Bacteria and Textile Fibers. — Many fibers, such as
ECONOMIC RELATIONS OF 0B0ANI8M8 301
those of flax, hemp, and others obtained from the stems of
plants, are obtained by rotting the stems until the surround-
ing tissues are soft. Bacteria do this work.
(f) Useful Decomposition, — While the action of bacteria
in decomposing foods prepared for human use is harmful
and financially a source of great loss, we should not forget
that this same process of decomposition is absolutely neces-
sary in order to prepare dead organic matter for use by
plants. We repeat for emphasis the statement made else-
where that but for the decomposing action of bacteria, aided
by other micro-organisms, the world would soon be so filled
with dead bodies of animals and plants that the carbon and
nitrogen needed in the food supply of living things would
be bound up in dead organic matter. Hence as decomposers
of organic matter the bacteria are indispensable.
A splendid example of the usefulness of bacteria in causing
decomposition is found in the latest methods for disposal of
sewage in concrete tanks or reservoirs arranged so as to allow
decomposing bacteria to grow rapidly. The result is that
sewage is changed so that even pathogenic (disease-produc-
ing) bacteria are killed. Many cities now purify their
sewage by this method before discharging it into rivers.
For private sewer systems in villages and in rural districts,
the best method consists in allowing sewage to run into
porous drain-tiles laid a short distance below the surface
in loose soil. In the soil around the tiles the decomposing
bacteria flourish and convert the sewage into harmless
substances which may be used by roots of plants. The
old-fashioned cesspools are being superseded by modern
bacterial methods, for cesspools do not favor rapid growth
of bacteria, and moreover they are often deep enough to
allow escape of dangerous sewage into subterranean water-
courses.
302 INTRODUCTION TO BIOLOGY
Summarizing the useful aspects of bacteria, it is evident
that their usefulness far exceeds their harmfulness, A few
individual organisms may die from bacterial diseases, but
the continuance of life on this planet depends directly upon
the decomposing action of the bacteria, which prepare organic
matter for use by plants. And quite apart from this all-
important aspect of our relation to bacteria, their value in
the other ways we have noted above far outweighs their
harmfulness as producers of disease. Moreover, in the not
distant future, when civilized people will carefully apply the
already well-known biological laws (§§ 249-257) so that
bacterial diseases will be kept under control, the usefulness
of bacteria will attract more popular attention than at
present, when the average citizen thinks of " germs " only
as producers of disease.
(6) MoUs*
Most of the useful aspects of bacteria are well treated in Lipmann's
" Bacteria in Relation to Country Life," and more briefly in Conn's
" Bacteria, Yeasts, and Molds in the Home."
272. Study of Common Molds. — It is a well-known fact
that when bread, cheese, or other foods are left for some time
in a damp place, there appears upon the surface a mass of
delicate threads or filaments, and the bread is said to be
" molding '' or " moldy." Soon the color of the moldy
mass changes from white to black, blue-green, or brown;
and according to the color, the names black mold, green
mold, and brown mold are applied.
* Molds, mushrooms, yeasts, and their allies are commonly grouped
under the name Fungi. They are all plants without chlorophyll, and hence
different in the food supply from the lowest green plants (Algae). For the
position of these plants in the classification of the plant kingdom, see table
in § 44.
ECONOMIC RELATIONS OF ORGANISMS
303
(L) Examine pieces of bread on which black mold is growing.
Break off pieces and note that delicate threads run through the
bread (use a hand-lens). These threads are called hyphas, and the
entire mass of them is the mycelium. Starting on the surface, the
mycelium grows numerous branches which penetrate every part of
the bread. At the same time it secretes digestive substances which
digest starch and other foods in the bread. These digested and
dissolved foods are absorbed by the mycelium and used for growth.
The delicate threads seen above the surface of the bread are
erect or aerial hyphsB ; they are branches of the mycelium. Close
examination with a strong
hand-lens will show that the
color of the mold is due to
spherical bodies attached to
the ends of the erect hyphsB.
These bodies are the spore-
cases, or sporangia, which
are filled with spores (Fig.
76, 8 ) . Various stages in the
development of the spore-
cases show that they are
formed by expansion of the
ends of aerial hyphsB. The
protoplasm inside these ex-
panded ends divides into
numerous small masses, each
of which becomes a spore.
Examine spores with high power of microscope. Notice the thick
cell-wall, which is very protective against heat, drought, and other
unfavorable conditions.
Examine green mold, especially noting the peculiar sporangia
(Fig. 77).
(D) If some spores be placed in dilute sugar solution in a watch-
glass, many of them will be found germinating after a day; and
after several days much branching will produce an extensive myce-
lium (Fig. 76). Later, some aerial branches or hyphaa arise and
form spore-cases. The entire mycelium and aerial branches with
spore-cases which originate from a single spore, is a mold plant
(Fig. 76). Usually such plants are closely interlaced, because
many spores fall on the same piece of bread.
Fig. 76. A black mold plant formed from
one spore in center. One mature atrial
hypha with spherical spore-case (s), and
two immature ones. Root-like structure
is mycelium. {From Kny.)
304
INTRODUCTION TO BIOLOGY
Fig. 77. Unlike the spherical spore-case of
the black mold, the green mold forms
spores (sp) at ends of branches of aerial
hyphse.
Molds grow best in a warm and damp place, as can be
prpved by leaving pieces of moist bread in two bottles, one
of which is placed near a stove or radiator, and one in an
ice-box. However, some molds will grow on foods kept for
some time in the ice-box.
In fact, many foods will
spoil in ice-boxes and
should be reheated to 100°
C. two or three times per
week.
273. Experiments with
Molds. — (D or L) Test-
tubes J by 5 inches are best
for these experiments, but
bottles with mouths at least
J inch in diameter will do. Cut some strips of bread J by 2
inches in size, and place one in each of ten or more tubes, and add
three or four drops of boiled water to each tube. Now, plug the
mouths of the tubes with cotton-batting or absorbent cotton, mak-
ing the plugs by rolling the cotton into a cylinder about two inches
long.*
Sterilization. — It is probable that both the bread inside the tubes
and the cotton have spores of molds on their surfaces, because
spores are so abundant in ordinary buildings that practically every-
thing exposed to air will have them. It is therefore necessary to
kill all spores inside the plugged tubes, that is, to sterilize or to make
the tubes sterile. This is best done by boiling or steaming (100** C,
2X2° F.), in. a sterilizer. There are many sterilizers for laboratory
and home use on the market, but one can be easily made from any
tin bucket with a cover. Place in the bottom of the bucket an
inverted tin pan two inches deep with holes in its bottom; and
on this, place the tubes to be sterilized. Put in water two inches
deep. For convenience in keeping mouths of test-tubes upward,
they may be tied into bunches, or placed in small tin cans perforated
with numerous holes. A layer of cloth or cotton between the glass
tubes and the metal will prevent breakage.
* To Teacher : See the "Applied Biology," § 245.
ECONOMIC RELATIONS OF ORGANISMS 305
Keep the water boiling (100° C.) for a half-hour. From time to
time add more water (hot) so that the sterilizer will not " boil dry."
Label some of the tubes " sterilized once, 30 minutes." Set
aside for observation from day to day. Do any molds appear on
the bread in these? Conclusions?
Sterilize the unlabeled tubes again two days after the first steriliz-
ing, and label " sterilized twice, total 60 minutes." Sometimes a
third sterilizing is necessary to kill all the spores of molds.
Inocvlaiion, — Take a tube which two or three days after the
sejcond or third sterilizing shows no sign of molds on the bread, and
hence is probably sterile, and inoculate it as follows : Sterilize an
inoculating needle (or a hat-pin, or piece of wire) by passing it
quickly several times through the flame of a gas- or alcohol-lamp.
Allow the needle to cool for a moment, and then touch the sterilized
end carefully to a spot on moldy bread where there is only one kind
of mold. Now, holding one of the sterile tubes in a horizontal
position, quickly pull out the cotton plug, insert the needle, wipe it
along one side of the piece of sterile bread, and then quickly replace
the plug. Inoculate other tubes from other spots of molds, of
different kinds if available, but take care to heat the wire of the
inoculating needle before and after each tube is inoculated, other-
wise you may mix several kinds of spores. Label tubes " black
mold," " green mold," etc.
Do not be surprised if with all these precautions two kinds of
molds appear in some tubes ; for spores may have been left alive
on the bread even after sterilizing, they may have fallen in when
the tube was opened for inoculating, or more than one kind of
spores may have been on the moldy spot touched with the needle.
If only one kind appears, a " pure culture " has been secured.
Take two tubes which have been steriUzed twice, remove cotton
plugs, and blow into them some dust from top of furniture, door-
frame, or window-casing ; or sterilize an inoculating needle and with
it transfer some of the dust to the sterile bread. Record result
and conclusions. Why do foods mold quickly in a dusty pantry?
Leave the plugs out of two sterile tubes for several hours, and
then replace. If molds develop, consider the possible source of the
spores. A better way for this experiment is to use a flat dish,
known as a Petri dish, made for such experiments (Fig. 78). Place
a slice of moist bread in such a dish, sterilize twice, wait several
days to make sure that the bread is sterile, then remove the cover
^H^A:^f±'i>»ii^fi:V,UM^
306 INTRODUCTION TO BIOLOGY
for several hours so as to expose the sterile bread to the air of the
schoolroom.
A small tumbler or wine-glass may be used' instead of a Petri
dish. Place a piece of moist bread inside on the bottom of a tum-
bler, cover with a circular sheet of cotton-wadding which by two
inches exceeds the diameter of the top of the
tumbler, fold the edges of the cotton down, and
snap a rubber band around it so as to hold it
FiQ. 78. Section of a close to the top of the tumbler. Sterilize as
Petri dish, for grow- in the case of the Petri dish,
ing molds and bac-
teria, c glass cover. Canning Fruit, — The glass j ars in com-
The dots m bottom ^ i i . . , . i.
show position of mon use are sealed air tight by means of
Sw food^*^* ^' ^ rubber ring. The above experiments
suggest that the rubber is not necessary.
The cap of an ordinary fruit-jar would protect the contents
against entrance of spores if the j ar and the fruit were thoroughly
sterilized after placing the cap in position. But in ordinary
household canning of fruits the sterilization (usually only
once) is not perfect and the keeping out of air by the rubber
tends to prevent the development of molds which require
air in order to grow. Also, the rubber prevents hyphae
from starting on the outside and growing beneath the cover
into the contents of the jar. This would surely happen if
fruit juice ran down around the edge of the cover, for the
juice would furnish excellent food for the growing hyphse.
This is why fruit in jars often remains sterile for months and
even years, and then suddenly shows molds. The rubbers
have softened, or loosened, or become moist, so that mold
hyphse have been able to grow into the jars ; or the entrance
of air may start growth of spores inclosed with the fruit.
Hence jars of fruit should be kept in a cool and dry place
unfavorable for growth of molds. Aside from this relation
to molds, rubbers of fruit-jars also prevent evaporation of
the fruit juices.
ECONOMIC RELATIONS OF ORGANISMS 307
Rotting of fruit has been mentioned as due to molds. Take a
perfect apple and inoculate it by sticking in several places with a
needle which has been plunged into a very rotten apple. Take
another perfect apple, bruise the skin on one side, and place it in a
covered can or box in contact with a rotting apple. In both cases
be sure there is moisture — the apples may be kept on wet cloth or
cotton. Take another perfect apple, gently rub the skin in several
places with a smooth stick which has been plunged into a rotting
apple, so as to distribute spores over the uninjured surface of the
apple, and set this one in a cool and dry place for comparison with
the others. What are the conclusions regarding apples infecting
each other? Why should injured apples be separated from perfect
ones before packing? Why are apples bored by worms (larvse or
codling moth) and " windfalls " so liable to decay? Why should
fruit-cellars be cool and dry ? Since the spores usually mature after
rotting is well advanced, it is evident that removal of windfall
fruits, as by pigs living in orchards, prevents the enormous multi-
plication and distribution of spores of various molds which injure
fruit.
Instead of pieces of bread called for in above experiments, a
dilute sugar solution (three or four tablespoonfuls of sugar in a pint
of water) may be used. Or use some diluted juice from canned
fruits. It is interesting tp prepare some tubes with the sugar
solution and try all the experiments both with sugar and bread.
Of course, any other organic material on which molds grow could
be used in place of bread or sugar solution.
Write a short essay on " Molds in Relation to Preservation of
Food for Human Use."
274. Allies of Molds. — Many fungi popularly known as
molds, mildews, rusts, rots, blights, smuts, scabs, — all of
them more or less closely related to the conmion molds which
grow on bread and other foods, — are very important eco-
nomically, because of their injurious effect on many cul-
tivated plants. Several special agricultural books are de-
voted to these economic fungi, and some of the common
varieties are briefly described in the " Applied Biology."
275. Useful Molds. — So many of the molds and their
308 INTRODUCTION TO BIOLOGY
allies are injurious from our human point of view that we
must guard against the erroneous impression that all rela-
tives of the molds are harmful. The following examples will
suggest some useful aspects of the life of these lower fungi.
The common molds which caUse decay oftentimes destroy
materials useful as human foods and in other ways, but the
same decay processes reduce useless organic matters to a
condition which makes them available for plant food. For
example, the vast quantities of leaves, stems, and other plant
materials seen in late summer must be decayed before their
elements can be used over again by plants, and in this decay
many of the decomposing fungi aid.
Moreover, some fungi are useful because they destroy
insects. *House-flies are sometimes seen with delicate white
threads projecting from their bodies. These threads bear
spore-cases and a mycelium grows inside the flies. The
spores which fall on other flies germinate, and so the disease
may soon become epidemic. Other similar fungous diseases
attack other insects, and millions of grasshoppers, chinch-
bugs, and other destructive species are thus killed.
It should be credited to the molds that some of them
produce the peculiar flavors of cheeses, such as Roquefort
and Camembert. The manufacturers take particular pains
to cause the growth of almost a pure culture of the proper
kind of mold.
But although some of the molds and their allies may be
useful from the standpoint of human interests, their useful-
ness is far overbalanced by their harmfulness, especially to
the numerous plants which are important to the human food
supply.
(c) Mushrooms
276. The term mushroom is here used as a general term
including all forms popularly known as mushrooms and toad-
ECONOMIC RELATIONS OF ORGANISMS 309
stools, for there is no scientific distinction between them.
Formerly all edible fungi were called mushrooms, and all
others toadstools; but many so-called toadstools are now
known to be edible. Practically and scientifically, then, the
two words are now used quite synonymously.
Structure of a Mushroom. — (L) Any common mushroom or toad-
stool will serve for this study. Specimens may be collected when
available and preserved in 5 per cent formaUn solution.
Examine a well-expanded specimen. Above ground it consists
of a stalk and a cap. On the under side of the cap in most species
are the gills, delicate plates radiating from the center. In some
species there are numerous holes instead of the gills, and in others
there are projections in the form of spines. If the cap be cut from
the stalk and left for a few days with gills downward on a sheet of
white paper, numerous dark-colored bodies (spores) will fall on the
paper, leaving a print of the under side of the cap and showing the
arrangement of the gills. Closer examination of the under side of
the cap will show the spores on the sides of the gills. Make sketches
in note-book.
Below ground the mushroom is attached to thread-like
structures which are popularly called the rootlets, but they
have no resemblance to real roots of higher plants, except
that they are in the soil. These thread-like " rootlets ''
are hyphcB and the entire mass of them is known as a myce-
lium (see mold, § 272). From time to time new mushrooms
or toadstools grow upward from the mycelium. At first the
young mushroom is a small, compact, rounded body, con-
cealed just below the surface until ready for expansion, when
it absorbs water rapidly and may reach its full growth in a
night.
Microscopic study shows the stalk and cap to be com-
posed of closely packed and interwoven threads or hyphse,
which have the same structure as the hyphse below ground.
The part above ground is, then, a sort of branch of the sub-
, terranean mycelium.
310 INTRODUCTION TO BIOLOOT
Reproduction of Mushrooms. — A new mycelium is pro-
duced by germination of a spore which falls in a favorable
place. The stalk raises the spore-producing cap above
ground so that the spores may be more widely disseminated.
Cultivated mushrooms are grown from " spawn " which
comes from seed dealers in the form of dry bricks composed
of partially decayed vegetable matter. The spawn is really
nothing but myceUum, either taken from old mushroom
beds, or started by scattering spores over materials which
are compressed and dried as soon as the spores have germi-
nated and formed an extensive mycelium. Such bricks
should be broken into pieces and planted in a bed of suitable
soil with abundance of decaying organic matter. Concern-
ing the cultivation of mushrooms, see Farmers' Bulletin No.
204 (free).
Physiology of Mushroom, — As described in § 62, plants
without chlorophyll are like animals in that they must get
their food from other organisms, either as parasites on living
things (e.g., dodder), or as saprophytes on decaying organic
matter. The common mushrooms are usually saprophytes,
but some of them grow as parasites on living trees {e.g.,
shelf-fungi).
In their respiration mushrooms are like animals in that
they take in oxygen and excrete carbon dioxide.
277. Lichens. — The familiar plants known as lichens,
which in the form of grayish scales or moss-like masses
grow on rocks and bark of trees, consist of two kinds of plants
Hving together in intimate connection. One of the plants
is a member of the Algae (§ 44), and able to make carbo-
hydrates from carbon dioxide and water ; while the other is
a fungus, which obtains its carbohydrate food from its
associate. The main mass of a Uchen is a dense mycelium
similar in microscopic structure to some mushrooms, and
ECONOMIC RELATIONS OF ORGANISMS 311
in this are the cells or filaments of the associated green
plant.
Such a living together is an example of symbiosis. A
similar association between a plant and an animal is that
of the green bodies in Hydra (§ 116).
Reindeer-moss (a valuable food for' reindeer) and Ice-
land moss (used in cooking) are lichens. Litmus (used for
testing acids and alkalies) comes from a lichen. On the
mountains lichens assist in weathering rocks, and thus form-
ing soil. An edible form found on sandy deserts is called
manna-lichen.
278. Economic Relations of Mushrooms. — Food Value,
The words " economic relations " will probably lead most
readers to think instantly of edible mushrooms. There are
many thousand species of the group to which the conunon
mushrooms belong, and many of them are edible. The fact
is that they have little value as human food, but they are
deUcious as relishes. Concerning poisonous mushrooms, see
§ 233.
Effect on Timber, — More important economically than
their food value is the destructive action of some mushrooms
on trees valuable for Imnber. The heart-wood is often
found filled with white threads (mycelium) which so soften
the wood as to make the lumber worthless. Hollow trees
are usually due to such decay. The mycelium seen in the
wood is connected with a shelf-like structure (often called
shelf-mushroom or shelf-fungus) on the bark of trees. In
walking through woodland one can easily find pieces of dead
branches with small toadstools attached, and if these be
split open, the white thread-like mycelium may be seen.
These threads (hyphae) enter trees through some injured
tissue, such as a broken branch, gnawing by animals, or
careless pruning which leaves large surfaces exposed. The
312 INTRODUCTION TO BIOLOGY
reason for painting all injured surfaces of trees with tar,
cement, or paint is that these substances will keep out mycelia.
Spores are abundant in the air and may at any time fall upon
an injured surface, germinate, and form a mycelium which
penetrates between the cells of the wood. It is an easy
matter to keep mycelia from entering a tree at an injured
place, but quite impossible to check them when once they
have penetrated deeply into the plant.
The " dry rot " that often attacks the foundation timbers
of houses and makes them unable to stand the strain of
supporting buildings is due to the mycelium of a near rela-
tive of conmion mushrooms. The remedy for ''dry rot"
is ventilation of spaces beneath buildings and coating tim-
bers with tar, crude oil, or creosote.
Some mushrooms have their mycelia penetrating the roots
of trees, causing decay and weakening so that heavy winds
uproot the trees.
Puff-halls, named because they puff out a mass of spores
when ripe, are near relatives of the conmion mushrooms.
Their mycelia grow in rotten logs and humus (decaying
vegetable matter). They contain numerous small cavities
in which spores are formed. Some of the pufif-balls are
edible in the young state.
(d) Yeast Plants
279. The Cause of Fermentation. — It is a well-known
fact that liquids containing sugar, such as juices of grapes,
apples, and other fruits, undergo a change called /ermento-
tion. The result of this process is the production of alcohol
and an invisible gas (carbon dioxide), which causes efferves-
cence of the fermenting liquid. No other method of pro-
ducing alcohol is known to occur in nature; and so for
ECONOMIC RELATIONS OF ORGANISMS
313
thousands of years fermentation has been used in making
wines and other alcoholic beverages from fruit juices. It
was not until the nineteenth century that the microscopic
yeast plants were recognized as the cause of fermentation;
and the experiments performed by the great French natu-
ralist, Louis Pasteur, between 1855 and 1865 will always be
famous as having placed our knowledge of fermentation
processes on a thoroughly scientific foundation.
280. Study of Yeast Plants. — (L) Scrape some particle^ from
a cake of compressed yeast, mix with a drop of water on an object-
slide, and gently lower a cover-glass into position. Prepare another
slide by mounting a drop of molasses-solution (water 10 parts,
common dark-colored molasses 1 part) in which some scrapings of
compressed yeast were placed on the previous day and the liquid
kept in a warm place (70° to 90° F.). It is best to draw up with a
rubber-bulbed pipette, or with a dipping tube, some of the white
sediment in the bottom of the bottle
containing the molasses-solution.
Examine the slides with high power
of microscope. Small oval translucent
bodies, some isolated, some united into
groups or chains, will be seen. Each
oval body is a yeast plant ; and it is
a single cell, consisting of protoplasm
with droplets of fluid and a nucleus
visible only when stained. The chains
are due to reproduction by formation
of buds. By comparing the sizes of
the cells in a chain, it is possible to
determine the order in which the buds
were formed (Fig. 79). The cells of a
chain finally become isolated ; and each
may grow into a chain of new cells
when food is available. An abundance
of long chains indicates that yeast has
been growing rapidly ; compare some taken directly from compressed
cakes with some which has been in molasses-solution.
Fig. 79. Yeast cells, two cells
beginning to bud, and three
chains of cells, formed by
repeated budding. Nuclei
not shown. Drops of fluid
are in centers of some cells.
(From Sedgwick and Wilson.)
814 INTRODUCTION TO BIOLOGY
Experiments voith Yeast, — (D) Experiments to show effect of
food upon growth of yeast, the effect of heat, the nature of the gas
evolved, the presence of alcohol in fermenting fluids, the preserva-
tive effect of dense sugar solutions, and yeast in bread-making are
outlined in § 251 of the " Applied Biology," and should be demon-
strated if time allows.
281. Physiology of Yeast Plant. — The plant is a one-
celled organism, without chlorophyll, and for food is depend-
ent upon sugar or starch built up by other plants. It is
therefore a saprophyte like the molds, mushrooms, Indian
pipe, and many other plants without chlorophyll. Pasteur
showed that yeast plants must have nine elements in their
food ; namely, carbon, hydrogen, oxygen, nitrogen, calcium,
sulphur, phosphorus, potassium, and magnesium. These are
necessary for the formation of new yeast protoplasm, that is,
for growth; and if fermentation is also to take place,
sugar (or starch convertible into sugar) must also be
present. Fermentation, which changes sugar into alcohol
and carbon dioxide, is due to an enzyme secreted by yeast
plants.
Yeast plants grow best at about 70° to 90° F. They are
quickly killed at the boiling temperature, and hence it is
quite easy to preserve sweet (unfermented) cider or grape
juice in bottles. Fermentable foods may be preserved at
the freezing temperature, which is colder than that of com-
mon ice-boxes.
Yeast plants can withstand partial drying in cakes of dry
yeast, but after some months all the cells are dead in such
cakes. Wild yeasts can live in the dust on fruits, or on the
sui*face of the soil in orchards and vineyards, ready to begin
growth as soon as the bruised or crushed fruit allows the
yeast plants access to the juices. Dry yeast cells float with
dust in the air, and this explains why sterile fruit juices
ECONOMIC RELATIONS OF ORGANISMS 315
(c.gf., boiled cider) of sugary solutions will soon ferment after
exposure to ordinary air.
282. Economic Relations of Yeasts. — It is only neces-
sary to allude briefly to thp vast industries of manufacturing
bread and alcoholic beverages which are dependent upon pro-
duction of alcohol and carbon dioxide by yeast plants. So
great is the importance of pure yeasts in these two industries
that enormous factories, costing immense sums of money,
are devoted to raising yeasts.
Apart from alcohol in beverages and in medicine, it has
long been invaluable in varnishes and numerous other com-
pounds used in mechanic arts. Recently it has been coming
into prominence as a cheap source of energy for engines,
burners, lamps, etc. It is possible to make alcohol cheaply
from much plant material which, on the ordinary farm, goes
to waste (unmarketable fruits and grains) ; and it is probable
that before many years pass the making of alcohol for me-
chanical purposes will be a vast industry in agricultural
regions.
Vinegary which is used so extensively as a condiment and
for preserving certain foods for human use, is best made
from alcohol derived from fermented fruit juice. For ex-
ample, yeast plants ferment the sugar of apple cider and form
alcohol ; and the vinegar bacteria, of which there are millions
in " mother-of-vinegar," ferment the alcohol into acetic
acid, which gives vinegar its acid qualities.
On the harmful side, we must mention the undesired fer-
mentation of fruits and sugary mixtures prepared for human
food, However, we have seen that heating to near boiling
will check such action of yeast. Sometimes yeasts cause
undesired fermentation in milk and cheese in dairies.
We must conclude, then, that on the whole, yeast plants
are decidedly useful as producers of alcohol and carbon
316 INTRODUCTION TO BIOLOGY
dioxide. Most of the bread-making since human civiliza-
tion reached a stage where attention began to be paid to
preparing the best possible foods, has depended upon yeast
plants. Quite apart from the question of the harmful use
of alcoholic liquors, to be discussed in the last part of this
book, there can be no question concerning the value of
alcohol for many other purposes. The usefulness of yeasts
forms a decided contrast to the harmfulness of the molds.
Reading: Section II in Coim*s " Bacteria, Yeasts, and Molds in
the Home."
283. Economic Relations of the Algae. — A large number
of simple plants that have chlorophyll are included in the
group Algae. The most familiar examples are the sea-weeds
and the greeii plants that form a delicate coating on the
walls of glass aquaria.* Some authors classify all the plants
that are lower than mosses in two groups, the Algae which
have chlorophyll and the Fungi which have not.
Probably the greatest value of the algae lies in the fact
that they make food for animals. This may be observed in
any aquarium where snails rasp off and eat the green algae
on the glass. Numerous small animals eat algae, and these
animals may serve as food for still larger animals. Immense
quantities of these low plants are found in the ocean as far
down as light penetrates. Even some large animals, e.g.,
certain fishes and whales, have strainers in their mouths
which enable them to collect large quantities of very small
organisms, some of them animals and some plants; but
especially do small animals which feed on simple algae serve
as food for the larger animals. It is now quite certain that
* To Teachers : Specimens of the Algae, such as the filamentous Spiro-
gyra or Vaucheria, the green coatings of glass aquaria that have stood ex-
posed to light, and dried or preserved specimens of sea-weeds, should be
exhibited for the sake of general acquaintance.
ECONOMIC RELATIONS OF ORGANISMS 317
directly or indirectly these simplest green plants play an
important part in the food supply of aquatic animal life,
much of which is of use to man. In this line the simple
algae have great economic importance.
Many kinds of large sea-weeds have economic uses.
Thus iodine is prepared from the ash obtained by burning
sea-weeds; and formerly this was also the chief source of
sodium carbonate, from which baking soda is made. Cer-
tain sea-weeds are used as food by Chinese and Japanese
and by the natives of the Malay Archipelago. Agar-agar
and Irish moss, both used in preparation of jellies, are made
from certain kinds of sea-weeds. Finally, in some places
near the sea, the sea-weeds cast up by the waves are used as
soil fertilizers. . It is now proposed to harvest with dredges
a species that is abundant along the Pacific Coast in order
to get its potash for agricultural use, but it is not yet cer-
tain that this will be profitable.
284. Uses of Mosses. — The mosses are higher than the
algae and fungi, but lower than ferns. They are common on
exposed and barren hillsides, in bogs, in cold northern regions,
in forests, and even in water.
One of the most useful mosses is the bog- or peat-moss
(Sphagnum) which grows luxuriantly where most other
plants cannot live, and its growth gradually fills the bogs
with dead vegetation called peat. This does not decay
rapidly because the water in peat-bogs is somewhat antisep-
tic (i.e., prevents the development of bacteria which cause
decay elsewhere). In Ireland and some other countries the
peat which is extensively used as fuel is composed largely of
species of peat-moss. The American peat contains many
other aquatic plants. The floating islands in some lakes
and the quaking soil in marshy regions are masses of dead
vegetation, often largely peat-moss, which have collected
318 INTRODUCTION TO BIOLOGY
some silt and thus formed soil on which other kinds of plants
grow. Sphagnum moss is extensively used for packing
plants for shipping; and in greenhouses for filling hanging
baskets for ferns and orchids, for mixing with soil to prevent
it from packing and to make it hold water, and for covering
the soil in flower-pots.
285. Economic Relations of Ferns and their Allies. —
These plants form a division called Pteridophytes. Many-
leaves and stems of fern-like plants are found as coal fossils,
splendid specimens of which are on exhibition at natural
history museums.
At the present time little use is made of ferns except for
ornamental purposes, for which their foliage is unsurpassed.
Many tropical species are kept in greenhouses, and several
species have long been favorite house-plants. Various kinds
of the related club-mosses (lycopods, not true mosses) are
also used in the same way. Some of them are called " ground
pine." The curious plant sold under the name of " resur-
rection plant " is one of the little lycopods.
One fern contains in its stem a powerful drug, known in
pharmacy as " extract of male fern," and often used for ex-
pelling tape-worms and round-worms from the intestines.
The horse-tails (equisetums) are relatives of the ferns.
They were known as " scouring rushes " in the pioneer days
in America, because their stems were used to scour pewter
and brass utensils. The scouring property depends upon
silicon particles in the stems (easily demonstrated by burn-
ing). Many of the equisetums, some tree-like, flourished
during the coal age (carboniferous). They are now conmion
in damp sandy places along railroads, ditches, and highways.
286. The economic importance of the seed-plants is so
vast that a special book would be required to discuss it
adequately. Here we can only point to the fact that the
ECONOMIC RELATIONS OF 0BGANI8M8 319
great majority of plants useful to man are seed-plants.
Practically all of our plant food-supply and that of our
domesticated birds and mammals; all of our forests useful
for lumber ; all of our plants which produce fibers (cotton,
hemp, linen, etc.) ; most of the plants which produce drugs
and other special substances; and almost all of our oma^
mental plants belong in the great gi*oup of seed-plants or
flowering plants. The mere mention of food plants, fiber
plants, and lumber calls to mind the vast agricultural and
manufacturing industries which have been built up on the
basis of seed-plants. In fact, the foundation of the wealth
of the civilized nations is in agriculture, which is primarily
the business of producing useful seed-plants. The useful
parts of seed-plants are ^eeds, roots, stems,, flowers, and
fruits. We have time for only a few examples of each of
these.
Seeds
287. Useful Seeds. — Quite apart from the functions of
seeds in propagation of new plant individuals, many of them
are useful to us because they contain foods, oils, and medici-
nal substances. Examples of seeds valuable in our food-
supply or for our domesticated animals are beans, peas,
lentils, com, wheat, rye, oats, barley, rice, buckwheat,
peanut, many true nuts, cotton, cocoa-bean, vanilla-bean,
coffee. Seeds with useful oils are : flax, furnishing linseed
oil used in paints ; castor-oil seed, whose oil is a lubricant.
Seeds with medicinal value are numerous; examples are
mustard, squash, castor oil, and anise. The great majority
of species of the seed-plants, or flowering plants, do not pro-
duce seeds directly useful to man.
320 INTRODUCTION TO BIOLOGY
Roots
288. Roots as Food. — In addition to their prime pur-
poses — anchoring plants to the soil and absorbing water
with certain plant foods in solution — many roots are directly
useful because they contain food for man and domesticated
animals. Examples are beets, mangels (a kind of beet),
turnips, ruta bagas (a kind of turnip), parsnip, radish, sal-
sify, sweet potato, carrot. However, the primary purpose
is not storage for man and animals, but for the plants' use
later. Many plants which thus store food in roots use it
in the second year for the development of flowers and seeds,
and then the plants die. Such are biennials or two-year
plants. Other plants which live many years (e.gf., rhubarb
and asparagus) store food in the roots every summer to be
used for early growth in the next spring.
289. Root-tubercles. — These are small rounded masses
found on roots of legumes (beans, peas, clovers, vetches,
alfalfa). Only by the aid of the bacteria which live in the
root-tubercles is it possible for plants to make use of the
nitrogen which constitutes over two-thirds of the air. These
bacteria have the peculiar power of combining nitrogen with
other elements derived from the soil, and the nitrogen com-
pounds thus formed are absorbed by the roots and used by
the plant just as it uses the nitrogen in manures and in such
soil fertilizers as nitrate of soda.
290. Parasitic Roots. — Many species of plants have
peculiar roots adapted for penetrating the epidermis of
other plants and thus forming a close attachment for absorb-
ing the necessary foods. Examples are the mistletoe, which
grows on the bark of oak, apple, and other trees ; and the
conmion dodder, which has root-like branches penetrating
other plants from which it can absorb suitable foods. These
ECONOMIC RELATIONS OF 0BGANI8MS 321'
are cases of plant parasites; and this condition in which
one plant (the parasite) absorbs its food from another plant
(the host) is parasitism. The mistletoe has pale green
leaves and can make some starch, so it is only in part a para-
site. The dodder has no roots in the soil, only some very
small scale-like rudiments of leaves without chlorophyll, and
a yellow, string-like stem with very little chlorophyll. Evi-
dently it must depend upon other plants for the foods which
ordinary plants get from the soil and make in their green
leaves. In recent years the dodder has been accidentally
planted in many fields by seed mixed with those of clover
and alfalfa, and has caused great damage by absorbing food
from the plants with which it makes a root-like connection.
For more facts concerning the relation of this weed to agri-
culture, read Farmers' Bulletin 306, " Dodder and its Rela-
tion to Farm Seeds."
291. Soil Binding by Roots. — After any heavy rain-
storm one can see that bare soil, such as that on roads and
cultivated fields, has been washed away where water has
flowed rapidly, while grassy sod has not been so affected.
This illustrates an important work of roots, that of holding
particles of soil together. Cutting a forest from a hillside
or sloping land allows rapid washing, and large tracts of
valuable land have been ruined by removal of fertile soil and
by formation of deep gullies.
The same kind of erosion often occurs when cultivated
fields are bare during the winter. Even if deep furrows or
gullies are not fohned, it is evident that the muddy water
which always flows down bare slopes is carrying away large
quantities of the fertile top soil. This has not been suffi-
ciently understood by many farmers in the past, but its im-
portance has been so clearly demonstrated that the scientific
farmer of to-day tries to protect soil against erosion during
322 INTRODUCTION TO BIOLOGY
the winter by planting some winter crop in late summer,
either after harvesting a cultivated crop like corn and vege-
tables, or between the rows after the last cultivation, or in
cultivated orchards and vineyards. This winter crop may
be wheat, rye, or grasses to be harvested in the following
sunmier, or a " cover crop " of certain clovers, vetches, rye,
and other plants which make great root growth in the autumn,
largely prevent erosion by the rains of the winter and early
spring, and add to the fertility of the soil if plowed under
in the next year's cultivation.
292. Propagation from Roots. — While many stems readily
develop roots, most roots do not form stems, and hence can-
not be used for propagation of new plants. Exceptions are
sweet potato, which is a thickened root, and roots of the
osage-orange tree. In order to start new plants of sweet
potato or osage-orange, it is simply necessary to bury root-
cuttings in favorable soil. A sweet potato will form stems
on any part of its surface ; but roots like carrot, beet, horse-
radish, and turnip soon die when the stem end or bud is
removed. Some plants (e.gf., dahlia) form a cluster of roots
at the bas6 of the stem, but each one of these can grow a
new stem only at the upper or stem end. Many of the root-
like structures from which new stems originate are really
underground stems (e.gf., the ordinary potato).
Stems
293. Useful Stems. — The most important stems are
those of trees used for wood, paper pulp, charcoal, and lum-
ber. Certain stems are used in our food-supply. Asparagus
plants above the soil are all stem, the leaves being reduced
to scales. The common potato is a thickened underground
stem stored with an abundance of starch. Much of our
ordinary sugar is made from the extracted juice of the stems
ECONOMIC RELATION ti OF ORGANISMS 323
of sugar-cane, but some is made from roots of sugar-beet.
Maple sugar is from the sap drawn from trunk (stem) of a
maple tree, usually the hard maple, but sometimes the soft
or red maple. Ginger " root " is really an underground stem.
Cinnamon is the bark of a tree. The edible parts of many
plants, such as spinach, onion, cabbage, celery, a^d the
grasses used as hay for animals, consist in part of stems
with the attached leaves. Among other stems whose prod-
ucts are useful are certain pine trees whose bark when in-
jured exudes tar, turpentine, and resin. Rubber comes from
certain species of trees belonging to the fig family. Quinine
is made from the bark of the cinchona tree. Hemp and flax
fibers are obtained by rotting the stems of the plants so as
to free the fibro-vascular bundles from surrounding tissue.
Straws used for hats and many other purposes are the stems
of rye, wheat, or other members of the grass family.
The value of many stems, such as those of trees grown for
lumber, fruit, or ornamental purposes, depends largely upon
their habit of growth and the consequent form and structure
of the stems. In order to understand many economic
problems relating to stems we must study them more care-
fully than we did in the first lessons on plants.
294. Twigs.* — This is a popular term applied to pieces
fif stems, or to small branches of trees and shrubs. They
are very convenient for study, because they illustrate on a
small scale the general structure of the entire stem of the
plant from which they are taken.
(L) Twigs of horse-chestnut, hickory, ash, box-elder, ailanthus,
linden, sycamore, etc. Note that the bud at the end of the twig
(terminal 6wd) is larger than the buds along the sides (lateral buds).
* To Teachers : If most of the students in the class have studied twigs
and buds in the nature-study of the elementary school, this exercise and the
following on buds should be given but brief attention.
324
INTRODUCTION TO BIOLOGY
Observe the positions of the buds along the stem (see Fig. 80).
Observe the large horseshoe-shaped scars immediately beneath the
buds. These are leaf-scars. What is the position of a bud with
reference to the leaf? Look for
Terminal Bud
Axillary Bud
Fruit Scat,
.lyeari growth
very small scars scattered irregu-
larly over the stem; they are
lenticels or breathing pores. The
nature of the rings of scars will be
made evident after a study of the
bud. Often one can find a large
scar in the fork of a horse-chest-
nut branch ; this is a flower-scar.
On a much larger twig, observe
the arrangement of the branches
— how does it compare with the
arrangement of the buds already
noted? This is what we should
expect if the branches are merely
developed buds, as stated in the
definition of a bud in the next
section.
Make a sketch of the twig
which has been studied, showing
as many of above points of struc-
ture as possible. Compare with
as many twigs as possible — lilac, apple, cherry, tulip-tree, butternut,
beech, and others named above.
295. Buds. — A bud is a growing point on a plant which,
when conditions are favorable, will develop into a leafy
branch, a flower, or both. In temperate regions where
plants must withstand a long, cold winter, trees and shrubs
have formed the habit of covering and protecting their
growing points with scales, hair, or gummy substances.
The horse-chestnut bud is a good example of a scaly or
vrinter bvdj and similar ones can be seen on any of our hardy
shrubs or trees. In annuals and herbaceous perennials of
temperate climates and in tropical countries where the grow-
FiQ. 80. Twig of horse-chestnut.
(From Blakeslee and Jarvis.)
ECONOMIC RELATIONS OF ORGANISMS 325
ing season is not interrupted by a winter season, with its
sudden changes of temperature and moisture, the buds as a
rule have either very little covering of scales or hairs, or
none at all {naked buds).
(L) Buds of horse-chestnut, hickory, lilac, elm, or any large
bud. Examine horse-chestnut huds and note that they are all
covered with sticky brown scales which overlap as do the shingles
on a house. Examine a bud cut longitudinally. Suggest a use for
the sticky substance. Carefully remove the scales from a bud and
observe the ring of scars left on the stem. This marks the end of
the year's growth of the stem. Look along the stem for other
similar rings of scars — how many do you find? What relation
does the number of such rings of scars bear to the age of the
twig?
Note that the bud-scales are arranged in pairs, and that they
become softer and more leaf-like as you approach the center. When
all the scales have been removed, there remains a soft, woolly object,
the young stem and leaves. Suggest a use for the hairy substance.
Sometimes the terminal bud is a flower-bud. Compare what you
have found in a closed bud with what you find in an opening bud
on a twig which has been standing in water in a warm room. Ob-
serve that such a bud has swelled, some of the outer scales have
fallen off, leaving a fresh ring of scars on the stem, the inner scales
have become larger and more leaf-like, the shoot has elongated, the
leaflets are separating and unfolding.
Compare buds from various trees and shrubs.
Leaves are formed folded in buds. This is necessary for
two reasons : (1) leaves in a bud must take up as little space
as possible ; (2) young, tender leaves must have as little sur-
face as possible exposed to the dry and cold air of winter.
The folding of the leaves in the bud is known in botany as
vernation.
Examine buds of various trees and shrubs such as lilac, tulip-
tree, hickory, beech, currant, etc., for types of vernation. Also
examine naked buds.
326 INTRODUCTION TO BIOLOGY
Bvd4ike Structures, — The cabbage head and the onion
bulb both illustrate the structure of a bud; but it should
be noted that they are not true buds, for they represent
fully developed shoots with very short stems.
(D) Cut lengthwise through center of an onion and a small
cabbage.
296. Position of Buds and Branches. — Some buds are
abnormal in position and are known as adventitious. They
develop on some trees when large branches have been cut off,
and advantage is taken of this fact by pollarding willow trees
and thus causing them to produce from adventitious buds
the straight and slender branches suitable for making baskets.
Some trees are pollarded in order to get ornamental trees
with rounded, bushy tops.
Opposite Branching, — When the buds are in pairs at each
node and opposite each other, there is a four-rowed arrange-
ment on the twig ; but owing to the failure of some of the
buds to develop and the death of some young twigs, this
arrangement is seldom perfect on fully grown branches.
As examples, observe maple, ash, horse-chestnut, and lilac.
Alternate Branching, — When the buds are arranged one
to each node, there may be two, three, five, or more rows.
In passing from any bud to the next above on the stem we go
spirally around the stem. Examples are hickory, elm, beech,
ailanthus, and walnut. Alternate is more common than
opposite branching.
297. Tree Forms. — If the terminal bud takes the lead
in growth, the result is a tree like the spruce or hemlock, in
which the straight main trunk can be traced to the very top
of the tree, and with relatively small lateral branches.
When the branches from the lateral buds develop rapidly,
the result is a round-top tree, like the apple, in which the
ECONOMIC RELATIONS OF ORGANISMS 327
main trunk cannot be distinguished from main branches.
Most of our common forest trees are intermediate between
these two types. Observe various trees and decide to which
of these classes they belong. Also notice that the character-
istic contour of a tree or shrub depends both upon its mode
of branching and upon the angle which the branch makes
with the main stem, e.gf., compare Lombardy poplars with
ordinary trees of our forests and parks.
298. Growth of Stems in Length. — Elongation is most
rapid just below the growing tip of stems and their branches.
If one measures the distance between marked branches on
a young fruit-tree in early spring and again in autumn, it
will be found that the older branches have not grown farther
apart or higher from the ground; but the branches which
appear during the growing season grow farther apart during
some weeks while the tissues are soft. This is a matter of
practical importance for one who cultivates trees. In prun-
ing, branches must be left at the height where it is desired
to have them when the tree is fully grown. Also, by cutting
the ends (terminal buds) of the main stem and of its branches
it is possible to control the height of the tree, and thus keep
fruit-bearing branches near the ground for convenience in
collecting fruit.
299. Growth of Stems in Diameter. — It is a well-known
fact that the trunks of our common trees continue to in-
crease in diameter. This is due to the formation of new
outer layers of wood by the cells of the cambium (§ 35).
New inner layers of bark are also formed by the cambium.
Commonly one layer is formed each year, and results in the
rings of growth seen in transverse sections of many trees
(Fig. 81). A hand-lens examination of a section of oak,
or other wood with clearly marked rings, will show that the
lines between the rings are marked by large wood tubes.
828 INTRODUCTION TO BIOLOGY
These are formed while the stem was growing rapidly,
usually in the spring.
The age of a tree cannot always be accurately determined
by counting the " annual '' rings, for it has been found that
sometimes two rings are formed in a single year. This may
happen if growth is checked for a time in early summer, as
by drought or destruction of leaves by caterpillars, and begins
again later in the summer.
The stems of plants whose structure is like the corn stalk *
have their fibro-vascular bundles arranged irregularly as in
Fig. 11, instead of in rings as in the ordinary trees. Such
stems do not increase in diameter each year by addition of
layers to the outside of the wood, but instead they grow
practically to the full diameter during primary growth, i.e.,
while elongating. Thus the trunks of palms and cocoanut
trees are nearly the same diameter almost to the leafy top ;
and although these trees continue to grow for many years,
there is little increase in diameter except near the growing
top. Such increase as does occur, as in some palms, is due
to expansion of the first-formed cells, not to new layers of
cells added between the bark and wood.
300. Structure of Stems of Common Trees. — (L) Materials :
Stems of oak, pine, beech, maple, and as many others as obtain-
able, selecting' stems of various diameters, from a half inch to two
inches or more. These should be cut off with a fine-tooth saw and
then smoothed by rubbing on sandpaper which has been glued or
tacked to a board. Also, some sections or cut-ends or twigs, one,
two, and three years old. In many high schools the boys who know
how to use carpenters* tools prepare a neat collection of woods.
Dead and well-dried (** seasoned **) branches are best.
The hand-lens shows that the annual rings are marked off by
large pores (the wood-tubes). Compare stems of varying diameters,
* The seed-plants which have stem structure like corn belong to the group
of the monocotyledons (§44), while the ringed type of stem is found in the
dicotyledons.
ECONOMIC RELATIONS OF 0BGANI8MS
329
and count the rings. Each season's growth begins with the large
tubes, formed when growing rapidly in spring, and extends outward
to the next ring of large wood-tubes. In many woody stems some
pith remains in the
center.
Most woody stems
have delicate radiating
lines, the pith-rays or med-
vUary rays, which are so
named because they are
composed of the same
kind of cells as those
found in the central pith.
Notice the difference
in color between the outer
and inner annual rings.
The lighter colored outer
part of the wood is the
sap-wood, so called be-
cause it is active in trans-
porting water and in
freshly cut stems appears
to be filled with sap.
Some of its cells contain
living substance. The
darker central wood is the heart-wood, composed entirely of dead
cells. How do you explain the fact that many large trees are
hollow (because the heart-wood has rotted or been burned away),
and yet they appear to be healthy trees ?
(D or L) Split blocks from stems of various kinds of trees (espe-
cially oak, chestnut, beech, ash, pine) through the center, and plane
the cut surfaces. The section thus made* is radial. Parallel to
the cut surface of one of the halves plane off the bark until some
heart-wood is exposed. This surface will be a tangential section.
In the radial section the pith-rays will appear as glossy plates of
wood, and in the tangential section it will be evident that each ray
extends some distance up and down the stem. Evidently the
delicate radiating lines or rays seen in the transverse section were
simply the edges of flattened plates which are placed vertically and
radially in the stem.
7/t *<iir m
Fig . 81. From four-year-old pine stem cut in
winter. Annual rings {l , 2, 3, 4) ; pith-rays
(m) ; cambium (c) ; bark with bast (6) ; lines
between wood of successive years (i) ; center
of stem (p). {From Strashurger.)
330 INTRODUCTION TO BIOLOGY
(D or L) Examine the inner and outer layers of bark in various
woody stems. Soften pieces of bark by long soaking and boiling
in water, tear them into pieces and observe the position and direction
of the fibers. Some of these are bast-fibers (elongated cells),
which have been mentioned as of great value in the textile indus-
tries, e.g. J flax (linen), jute, ramie, and hemp are all obtained by
rotting the bark of certain plants and then separating the bast-
fibers.
301. Grafting and Healing Wounds on Stems. — The
power of growth of the cambium is well illustrated in these
two processes. If two branches of a tree be scraped down
to the cambium and the wounded surfaces fastened together,
the rapid growth of cambium cells will cause a grafting or
union of the two. Examples of this are sometimes found in
a forest or orchard, for branches often grow so near together
that the swaying by the wind wears away the bark, exposes
the cambium, and allows the two branches to grow together.
Two small trees may so grow together.
The observation that stems will grow together or naturally
graft probably led to artificial grafting of fruit trees. Essen-
tially all grafting consists in bringing the cambium layer of
a piece of twig with a bud (scion) from one plant into contact
with the same layer on a stem of another plant of the same
or sometimes a closely related species. Various ways of
doing this are illustrated in Fig. 82. Grafts are usually
placed on young plants near the roots (" root-grafting ")»
so that after the graft growsr and the original stem is cut
away only the variety of the graft will be represented in the
stem above ground. Old trees are grafted by cutting off
branches (^^ top-grafting ") and inserting scions or grafts.
If a new branch grows below the graft, it will bear fruit of
the original variety.
Budding is essentially the same as grafting, but a single
bud with a slice of bark is placed in contact with the cam-
ECONOMIC RELATIONS OF ORGANISMS
331
bium of another tree, and after
the bud grows into a stem the
original stem is cut away (Fig.
82, lower row).
The growth of new tissue
which takes place in the heal-
ing of wounds, as after prun-
ing or breaking of limbs by
wind, is essentially the same
as in grafting. A ring of new
tissue (callus) forms at the
bark and tends to grow over
the entire wound.
In cases where very large
limbs have been cut off, the
wood may decay before the
cambium has been able to form
enough new tissue to cover
the wound completely. The
remedy is (1) prevention of
decay by painting or tarring
all injured places so as to
keep out the bacteria and
molds which cause decay ; and
(2) if decay has already
started, the softened wood
should be cut out and the cav-
ity filled with tar or water-
proof cement.
302. Pruning of Stems.
— Unnecessary and injured
branches of trees, shrubs, and
vines should be cut off. Un-
FiQ. 82. Upper row — whip- or
tongue-grafting, a, stem or stock;
b, scion or graft; c, united and
wrapped. Middle row — cleft
grafting, a, scion; 6, two scions
inserted in cleft; c, waxed. Lower
row — budding, a, stem cut to
receive bud (6); c, bud inserted
beneath bark; d, wrapped. Note
in all the figures that cambium of
scion and stock meet. (U. S. Dept.
Agriculture.)
332 INTRODUCTION TO BIOLOGY
pruned fruit trees develop too many branches, and grow too
high for convenience in spraying against fungi and insects
or in picking fruit.
Pruning is now so well understood that a gardener can
make a young tree take almost any desired shape by simply
removing buds and branches which are not wanted, and
thereby forcing growth in other directions. For example,
pruning off a terminal bud makes the lateral branches grow.
See Farmers' Bulletin No. 181.
Nature's method of pruning by crowding out superfluous
branches is well illustrated in any dense wood. The shaded
branches soon die and decay, and the result is the growth of
long, straight stems with living branches near the top only.
Why are trees set close together in young forest plantations?
Pruning trees and shrubs at planting time is of great value,
in that it reduces the number of leaves until the roots can
get well established in the soil. Unless the leaf-surface is
thus reduced, evaporation is likely to be greater than can
be replaced by absorption of water by the roots. It is diffi-
cult to transplant evergreens, because pruning would spoil
the form of these ornamental trees, and the leaves cause ex-
cessive evaporation before new roots are established in the
soil.
303. Duration of Life of Plant Stems. — It is a matter of
common observation that some plants live only one year
and that others live many years. In many cases this is of
great economic importance. All plants of which the life of
a generation, beginning with the seed and ending with death
of stem and roots, comes within a year are called annuals.
That is to* say, an annual plant is one which completes its
life-history by starting from seed, producing seed, and dying,
root and all, within a single year. Other plants (e.g., beet,
cabbage, thistle, mullein) start from seed one year, the roots
ECONOMIC RELATIONS OF ORGANISMS 333
and base of the stem live over the winter, and in the second
summer they blossom, produce seed, and then die. Such
two-year plants are biennials. Still other plants (especially
trees and shrubs) live more than two years ; and such plants
are perennials. Some of these have a soft stem which can-
not withstand the winter, but the underground parts are
hardy and new stems form above ground in each growing
season. Examples are peony, golden-rod, hollyhock, as-
paragus, violet, dandelion, buttercup, and most common
grasses of our lawns and meadows. Such plants are called
herbaceous perennials. Many of them are short-lived; for
example, the best crop of red clover is grown in the first
and second years, and after that the roots are unable to
produce luxuriant growth of stem and leaves and gradually
die out, giving place to grasses and weeds.*
304. Underground Stems. — Many plants have their
main stems underground. Three common forms of such
stems are rootstocks, tubers, and bulbs.
Rootstocks. — These are root-like stems which grow in
the soil. Many ferns, grasses, sweet flag, golden-rod, quack-
grass, peppermint, iris, Solomon's
seal, and trilliums are examples of
common plants with rootstocks (rhi-
zomes). That these are not true
roots is shown by the presence of buds
and by the formation of branches Fig. 83. Underground
, , CI 1 > 1 /TT oo\ ste™ of Solomon's seal.
and leaves. Solomon's seal (Fig. 83) „. terminal bud; 6, c, d,
is especially good for study. Its «» leaf-scare of former
rootstock sends up an erect branch y^^^^* **» ^^^
every spring which becomes the above-ground stem with
leaves. All the above-ground part of the plant is herbaceous
* The duration of other useful plants is considered in § 168 of the *' Ap-
plied Biology."
334
INTRODUCTION TO BIOLOGY
and dies in autumn. The seal-ljke scars on the rootstock
mark the positions of above-ground branches in successive
years. The oldest portion of the rootstock dies and decays ;
and some plants have short rootstocks, because the part
which is more than a few years old has died and decayed.
Tubers. — Many of the rootstocks mentioned above store
food, but do not undergo noticeable thickening at any par-
ticular point. In some plants storage of food in undergroimd
stems causes great enlargement and produces tubers (e.g.,
the common potato and Jerusalem artichoke).
(L) Examine a tuber of potato, or artichoke. Note the point
where it was attached to the main stem of the plant. Examine
the " eyes," which are buds.
Each " eye " is capable of de-
veloping a new plant, and
hence to avoid having too
many plants in one place gar-
deners cut' tubers into pieces,
each having two or three
** eyes." Dig up a potato
plant, and note position of the
old and the new potatoes.
Bulbs, — The onion is a
good example of a bulb. A
longitudinal cut through the
center shows it to be a short
stem surrounded by thick
layers of modified leaves.
Indian turnip (Jack-in-the-pulpit) and crocus have similar
short and bulb-like stems, but they are solid and not composed
of layers like the onion. Such solid, short, underground
stems are often called corms ; but there is no sharp distinc-
tion and corms are often called solid bulbs.
^' Stemless " Plants, — Many common plants (e,g,, dande-
Fio. 84. Potato plant, developed from
old tuber in center. The new tubers
are thickenings of underground
branches of the stenr. (From Stras-
burger.)
ECONOMIC RELATIONS OF ORGANISMS 335
lions, plantains) have short stems and are often incorrectly
said to be " stemless.''
306. New Plants from Stems. — Strawberries, red rasp-
berries, currant, and gooseberry are examples of numerous
kinds of plants which have some branches either lying on
the surface of the soil or creeping underground, and which
form roots and develop new plants. When these branches
are underground, gardeners call the new plants which come
up " suckers."
306. Economic Value of Wood Structure. — The useful-
ness and consequent monetary value of lumber depend
upon the nature of the various structures seen in the sections
of stems. For many purposes hardness and strength com-
bined are desired (e.g., white oak for wagon-axles, hickory
for ax-handles and wheel-spokes). Sometimes a light,
elastic, straight-grained wood is wanted ; and so ash is most
conmion for the long handles of hoes, shovels, pitch-forks,
etc. Woods which resist decay because they contain resins,
oils, or other protecting substances are needed for fence-posts
and telephone poles ; and for such purposes cedar, chestnut,
oak, mulberry, and black locust are usually preferred.
White and red cedar, cypress, and redwood make the best
shingles now in the market. For some purposes wood
softer than oak is desirable because easy to work — car-
penters charge much more for working oak than for soft
woods like white pine. Hence pine, hemlock, spruce, red-
wood, basswood, tulip-wood or whitewood, cedar, sweet gum,
and poplar are used for numerous purposes where soft and
light wood will serve. For high-grade furniture, woods with
beautiful grain and color combined with hardness are de-
sired ; and hence the popularity of mahogany, black walnut,
rosewood, ebony, and oak.
It often happens that in a given locality a wood is used
336 INTRODUCTION TO BIOLOGY
for a purpose because it is the cheapest available, but not the
best. For example, hemlock is largely used in eastern states
for frames of buildings ; but its softness and tendency to split
make it far inferior to spruce among soft woods and not to
be compared with the oak, formerly much used for this pur-
pose. White pine and spruce are often used for cheap floors,
but they are as. much inferior to yellow pine as this is to hard
maple and oak for this purpose. Such semi-hard woods as
elm, chestnut, birch, and basswood are now abundant in
cheap furniture, and are often stained to imitate harder
woods like oak and mahogany.
Woods like chestnut and red oak, with large wood-ducts,
making the annual rings very conspicuous, are said to be
" open-grained.'' In finishing such woods for furniture it
is first necessary to use wood-fillers, which are pastes for
filling the open spaces left after planing, and thus give a
perfectly level surface to receive the varnish. Examine
polished oak furniture and compare with an oak board which
has been planed but not filled.
A wood which may be split into straight pieces parallel
to the central axis of the stem is said to be " straight grained."
Ash, chestnut, and hemlock are commonly so; but irregu-
larity in thickness of annual rings and in position of the
medullary rays, vascular bundles and knots makes some
woods more or less ^' cross-grained"; that is, they tend to
split obliquely and not parallel to the central axis of the
stem. Many woods (such as elm, sycamore, apple) are diflS-
cult to split because their wood-fibers are crossed and in-
terlaced. Split some pieces of boards or branches of various
trees by driving a wide chisel carefully and notice the direc-
tion of the fibers of the wood.
The usefulness of boards for certain purposes depends
upon the direction of sawing. Examine pieces of pine,
ECONOMIC RELATIONS OF 0BGANI8M8 337
cypress, maple and other boards, especially where they have
been subjected to wear, as in floors, and note the direction of
the cut of boards which have splintered or in other ways be-
come undesirable. The transverse section is best if a block
is to be subjected to great strain, as in pavement-blocks,
mallets, etc.
The decorative value of the grain of woods depends in
part upon the way the logs are sawed into boards. Compare
the tangential and radial sections of oak and other woods,
and decide which is the most beautiful cut.*
307. Forestry. — The application of scientific principles
to the management of forests is known as forestry. Such
scientific management has become necessary because the
use of tree stems as lumber has greatly reduced the extent of
forests ; and in the interest of future supply it is now desirable
that forest planting and forest conservation be practiced
extensively. The United States Forest Service has charge
of the national forest reserves, and also encourages private
work in forestry by publishing pamphlets and by giving
advice. Among the best of such pamphlets now available
are the " Primer of Forestry " by Pinchot, which is in two
parts; Farmers' Bulletins, and many leaflets (also free)
giving information as to how to plant special kinds of trees ;
and a pamphlet " Forestry in Nature Study." All these
are issued free by the United States Department of Agri-
culture. Another interesting book in the same line is Roth's
" First Book of Forestry/'
Leaves
308. Useful Leaves of Seed-Plants. — In addition to
their necessary work connected with the breathing, transpira-
*To Teachers: See § 179 and Figs. 55, 56 in^ the "Applied Biology,'
and explain the methods of ordinary and "quarter" sawing.
338 INTRODUCTION TO BIOLOGY
tion, and carbohydrate-making of all important higher plants
(§ 43), leaves have a number of special uses which are of
economic interest. These are food, ornamental, and medici-
nal values.
Celery, cabbage, onion, chard, lettuce, spinach, Brussels
sprouts, rhubarb, endive, cress, and parsley are examples
of vegetables grown chiefly for the food value of their leaves.
The same is true of the hay and forage obtained from such
plants as grasses, clovers, alfalfa, corn, and others which are
edible for domesticated animals.
Among leaves that contain medicinal substances are tea,
camphor, catnip, sage, mint, horehound, tansy, and many
others.
The ornamental value of leaves has been carefully studied
by those in charge of parks, gardens, and greenhouses ; and
plant breeders have succeeded in selecting a large number
of varieties of plants whose value is due to their leaves hav-
ing peculiar shapes, beautiful arrangement, or striking colors.
Many plants grown in homes or in greenhouses are good
illustrations in this line; e.gf., compare colors and forms of
leaves of varieties of coleus, geraniums, or begonias.
Flowers
309. Useful Flowers. — In addition to their necessary work
in the formation of seeds, some flowers are of special value
to man in the following ways. The flowers of cauliflower
and French artichoke are important as edible vegetables.
Many flowers contain the same medicinal substances as the
leaves and both are collected at the same time for the pur-
pose of making medicines. From certain flowers (e.g., rose,
violet, heliotrope) well-known perfumes are extracted.
Many flowers, especially of buckwheat, melilotus, clovers,
ECONOMIC RELATIONS OF ORGANISMS 389
and fruit trees, are useful as sources of honey which is col-
lected by the domesticated honey-bees.
Most important of the values of flowers is their ever-
popular use for ornamental purposes. This use is increasing
rapidly, probably because people are learning more to ap-
preciate beauty in nature. The very fact that millions of
dollars are spent each year in growing many kinds of plants
for the sake of their flowers is sure proof that flowers are
regarded as useful by thousands of people. School-gardens,
nature-study, and biology in the schools are doing much to
increase the use of flowers for ornamental purposes. It will
interest many students who have never examined a seeds-
man's catalogue to look over the pages devoted to flowers.
One of the most interesting facts is that under cultivation
so many remarkable flowers have been developed. This has
been done by preserving for propagation any plants which
chance to develop unusual flowers. For example, California
poppies are naturally yellow, but some years ago a horti-
culturist found a plant with crimson flowers, and now there
is for sale plenty of seed of the crimson-flowered variety.
In the same way the plant breeders have obtained the numer-
ous varieties of flowers catalogued by seedsmen. Perhaps
the most wonderful are the ^' double " ones, in which the petals
have become very numerous and the stamens and pistils
have largely or entirely disappeared. Many varieties of
cultivated roses and chrysanthemums are " double." It is
obviously impossible to raise completely " double '' flowers
from seeds, for without stamens and pistils they would be
seedless. Such varieties must be propagated from cuttings.
Fruits
310. Useful Fruits. — The word fruit is popularly applied
to the edible products of certain flowers, such as apples.
340 INTRODUCTION TO BIOLOGY
pears, berries ; but in botany fruit includes all the structures,
usually containing seeds, which develop from flowers, chiefly
from the ovary. Thus a bean pod, a walnut with its hull,
a chestnut with its bur, and a tomato, are fruits in the botani-
cal sense.
Many fruits are useful for the seeds that they contain,
but these have been referred to under seeds. There is a long
list of edible fruits — such as apple, pear, quince, cherry,
plum, peach, mulberry, apricot, nuts, berries, fig, pods of
bean and pea, orange, lemon, banana, date, pineapple, grape,
egg-plant, garden pepper, squash, pumpkin, melon, and
cucumber. From some fruits medicinal substances are ob-
tained ; e.g.y opium and morphine from the poppy. Various
bright-colored fruits were once used for dyeing, but the anilin
dyes have largely taken their place. Descriptions of various
types of fruits are given in §§ 212-217 of the "Applied Biol-
ogy/'
II. ANIMALS OF ECONOMIC IMPORTANCE
311. Microscopic Animal Parasites of Domesticated
Animals. — A protozoan (§115) parasite similar to that which
causes sleeping sickness in humans (§259),' and injected by
a similar fly, causes the disease of horses, mules, and oxen
called " nagana '' by the Africans, and by them correctly
charged to the tsetse-fly. Those who have read the" Travels"
of Livingston and other African explorers will recall how their
journeys were so often delayed by death of their oxen. The
disease also caused much trouble among horses during the
South African war a few years ago. The fly does not develop
the parasite, as mosquitoes do in the case of the malarial or-
ganism, but simply transmits the parasites from diseased to
healthy animals. It is believed that the blood of some large
game animals may infect the flies.
ECONOMIC RELATIONS OF ORGANISMS 341
The surra disease, which destroys large numbers of horses,
camels, and cattle in India and the Philippines, is due to a
similar parasite transmitted by bites of flies.
Destructive diseases, caused by protozoan parasites which
are introduced into the blood by the bites of ticks, affect
cattle, dogs, horses, and sheep. The parasites enter the red
blood-cells. Texas fever is one form of cattle disease, and
it has caused enormous financial losses to stockmen in the
United States.
A common disease of the liver in rabbits, diseases which
seriously affect fish, the silkworm disease made famous by
Pasteur's studies, — these are other cases in which parasitic
one-celled animals have great economic importance.
312. Useful Protozoa. — The emphasis upon certain
protozoans as causes of disease tends to leave the false im-
pression that all protozoa are harmful, or at least useless.
A large number of common protozoans are important in
the food-supply of somewhat larger animals, these in turn
of still larger animals, and so on to aquatic animals such as
fishes.
Thus even the minute bacteria eaten by some protozoans
may indirectly, through a series of animals of increasing size,
finally come to be of use in the food-supply of man himself.
Many of the protozoans are important as scavengers,
assisting bacteria in breaking up dead organic bodies and
preparing the organic material for use again in the cycle of
organic matter. For example, a protozoan may eat some
particles from the body of a dead animal or plant, build these
particles into its own protoplasm, which may later serve as
protein food for some larger animal ; or these particles may
soon be oxidized to excretions which may serve as materials
for food of plants.
Another example of the usefulness of protozoans is that a
342 INTRODUCTION TO BIOLOGY
large amount of chalk in the great deposits in England and
elsewhere is composed of the shells of certain protozoans.
In some places these shells are very abundant in the mud of
the ocean bottom.
313. Useful Sponge-Animals. — The simplest in structure
and therefore the lowest animals whose bodies are composed
of many cells are the sponge-animals, so called because their
skeletons are the sponges of commerce. Most of them are
marine animals, but certain species of no commercial impor-
tance live in fresh water. Of the marine species, some secrete
skeletons of the homy material found in the sponges sold in
stores; but some have skeletons of glassy fibers and some
are calcareous. The glass sponges are collected and sold
as curios, while the calcareous sponges are of interest to
zoologists because they represent the simplest type of sponge
structure. One who wishes to understand how the cells of
the sponge-animals form the complicated mass of fibers
which we call sponges should begin with study of some of
the simple sponges, as outlined in § 281 in the " Applied
Biology," or any high-school textbook of zoology. The
sponge-animals belong to the group Porifera (§ 87).
314. Coral- Animals and their Allies. — The sponge-animals
are the simplest of the many-celled animals ; and next above
them are the coral-animals and their near relatives of the
group Coelenterata (coelenterates). Perhaps the coral-ani-
mals and the jelly-fishes are the best known members of the
group. We have already studied the Hydra, because it
well illustrated certain points of physiology. The great
majority of the coelenterates are not of economic interest;
but they are very attractive to students of zoology in the
last years of the high-school course or in college, because
they have many remarkable structures and life-histories.
Most important to man of all the allies of Hydra (§116),
ECONOMIC RELATIONS OF ORGANISMS
343
are the animals whose skeletons form a large part of many
coraf islands. (Look in a textbook of geography for a list
of coral islands.) Sea-anemones resemble coral-animals in
structure, but do not form
skeletons.
^!«
(D) Specimens of sea-
anemones should be exhibited,
and if some are available for
making transverse and longi-
tudinal sections, follow the de-
scription in § 129 of the ** Ap-
plied Biology.**
Formation of Coral Skel-
etons, — On every piece of
coral which has not been
corroded by water, there
may be seen numerous cup-
like depressions, each with
many radial partitions. In
each of these cups there
was once located a coral-animal, or coral-polyp, which
secreted the cup. Imagine a sea-anemone able to form
a calcareous skeleton around itself, and then you can under-
stand the relation of a coral-animal to its surrounding cup
(Fig. 86).
The fact that a piece of coral shows may cups is explained
by the multiplication of individuals by budding. A coral
animal which develops from a fertilized egg-cell settles
down on a rock or on skeletons of preexisting corals and
begins to secrete a skeleton around itself. Buds are formed,
which do not become detached as in Hydra, and these begin
to secrete skeletons. The final result of oft-repeated budding
is a complicated calcareous mass, in tree-like or hemispherical
Fig. 85. Common sea-anemone con-
sisting of a cylindrical body with a
crown of numerous small tentacles
surrounding the mouth. (After Em-
erton.)
344
INTRODUCTION TO BIOLOGY
Fig. 86. A piece of coral show-
ing cups formed by eleven indi-
vidual polyps. (From Dana.)
form, with numerous cups representing the number of polyps
which took part in the formation.
The sea-fans and sea-plumes
are coral skeletons formed in an-
other way. Their outer surfaces
are calcareous, and their central
axis is composed of a horny mate-
rial. This is black in many species,
but red in the case of the pre-
cious coral from the Mediterra-
nean, which is used in jewelry.
One species of coral-animal is
found in Long Island Sound, but
most of them require warmer waters. They can live at
any depth down to about 300
feet, but are seldom found below
120 feet. They must have clean
and undiluted sea-water. Most
conmionly the young animals
developed from eggs attach
themselves to favorable sea-
bottom near land, and form
fringing reefs of coral rock near
the shore, or barrier reefs when
a navigable channel is eroded
between the reef and the shore.
The formation of an atoll, a
peculiar coral island in ring form
and inclosing a body of water,
may begin as a fringing or barrier
reef around a volcanic island
which later subsides or is eroded away while the deposits of
coral rock continue accumulating. Or a small group of cor-
FiQ. 87. A colony of coral-ani-
mals whose supporting "skele-
ton " (black in figure) contains
the red coral of commerce.
(After Lacaze-Duthiers.)
ECONOMIC RELATIONS OF ORGANISMS 345
als may become established on the sea-bottom at a favorable
depth, and as the colony grows outwards, the animals in
the center are killed by coral-sand washed over them by
the waves. Erosion takes place rapidly when there are no
living coral-animals to keep adding to the coral rock ; and
so the central rock might be worn away, leaving a ring of
living animals continually adding new coral rock to the
outside.
315. Parasitic Worms in Domesticated Animals. — Flat
worms, like the tape-worms, and round worms, like the
trichina (§ 264), are parasitic in pur conmion domesticated
animals and interfere with their health. The tape-worms
may occur either as the larval stage of the species which as
an adult lives in the human intestine (§ 263), or the adult
worm of certain species may live in the animal's intestine.
For example, the tape-worms found in the intestines of dogs
are believed to have their larval stage in the fleas that live
on the skin of the dogs.
One of the destructive diseases of sheep is caused by a
worm known as liver-fluke, which lives in the bile-ducts.
It is a flat worm about one inch long and one-fourth inch
broad, with two suckers for attachment. The fluke's eggs
escape into the sheep's intestine through the bile-duct, and
after being discharged from the intestine, each egg develops
into a larva covered with cilia. This larva swims, and if it
reaches a pond-snail, bores into it and becomes an elongated
sac. Inside this sac are formed many new larvae, and inside
each of these larvae are formed many more. Thus one larva
entering a snail produces a large number of larvae ; and each
one of these may leave the snail, become encysted on grass,
and if eaten by a sheep, will enter the bile-duct and develop
into a liver-fluke. When once a damp pasture along a given
stream has become infested with larvae of liver-flukes, the
346 INTRODUCTION TO BIQLOQT
only way to avoid infecting sheep is to keep them away from
that stream for a number of years. Ultimately the fluke
larvsB would disappear from a pasture if there were no sheep
in which the parasites could complete the life-history.
Many species of round worms are parasitic in domesticated
animals. Some of them live in the alimentary canal, others
in blood-vessels, respiratory organs, kidneys, and elsewhere.
Horses and cattle are believed to get the young worms of
certain species from stagnant water ; and this suggests the
importance of pure water for domesticated animals. The
so-called " gapes " of young poultry is caused by a round
worm which fastens itself in the trachea.
316. The Earthworm. — This common worm, also known
as " fishing worm," deserves more than the brief attention
which our time will allow. It lives in moist soil which con-
tains decaying organic matter, crawls out at night to feed,
or when its burrows are flooded with water. It eats the soil
through which it burrows, and its digestive juices dissolve
bacteria, leaf-mold, and other organic matter contained in
the soil. The indigestible soil is discharged and forms the
" castings " which are abundant on the surface of soil where
the earthworms live. Darwin found places where he esti-
mated that worms brought more than 35,000 pounds of soil
per acre to the surface in a year. This continual working
of soil by worms is regarded as of great agricultural value.
General External Structure, — (L) Notice a living worm as it
moves, which it does by successively elongating and shortening its
body. Determine anterior, posterior, dorsal, and ventral. Com-
pare the color on the dorsal and ventral surfaces. Is the animal
bilaterally symmetrical? Estimate the number of segments or
rings in the body. Locate the mouth and anus. Notice the
glistening surface of the skin due to a cuticle. About one-third or
one-quarter the body length from anterior end, there is on adult
worms in the spring a swollen region, the girdle. It secretes a
ECONOMIC RELATIONS OF ORGANISMS 347
oocoon in which the eggs are laid. Four double rows of small bris-
tles (setflB), which aid the worm in locomotion, may be located by
pulling a preserved specimen between a thumb and a finger, and
by using a hand-lens.
The dorsal and ventral blood-vessels may be easily seen in a
living animal. Select a light-colored worm, place on a moist glass
plate, and then lay another piece of moist glass on top of the worm
so as to compress the body slightly.
Internal Organs, — {D optional) The teacher should point out
on preserved earthworms, which have been cut open along the dorsal
side and pinned out on a narrow board, the systems of organs as
described in the " Applied Biology," § 302.
317. Economic Importance of Crustacea. — The most
valuable of the larger Crustacea is the North American
lobster, which lives on the ocean bottom within fifty miles
of the shore along our North Atlantic Coast, and the Nor-
way species, which is supplied to European markets. The
large size and the peculiar qualities of lobster flesh have led
to such a market demand that there has been excessive catch-
ing in recent years. The market value of the lobsters caught
each year is estimated to be many millions of dollars. In
recent years it has become evident that steps must be taken
to prevent extinction of the American lobster. Accordingly,
the United States Bureau of Fisheries has engaged exten-
sively in hatching lobster eggs, and several states are en-
forcing laws which prohibit taking short lobsters (under six
inches long in some states, nine in others) and females with
eggs attached to the abdominal appendages. The advan-
tage in hatching lobster eggs artificially is due to the fact
that fishes and other enemies usually destroy a very large
number of eggs or young larvae, but in the hatching troughs
they can be protected until several days old and better able
to care for themselves. The eggs are collected by brushing
them from the abdominal appendages of the females. A
348
INTRODUCTION TO BIOLOGY
female a foot long may have over ten thousand eggs, and
giant specunens over eighteen inches long have been found
with more than 150,000 eggs.
Next to the lobster, the crabs are important as articles of
human food. The blue crab
is the favorite. Spider crabs
and fiddler crabs are not
used as food. The " soft-
shelled'' crabs of our markets
are simply individuals which
have recently shed their
shells (i.e., molted), and
the new shell has not had
time to harden by deposit
of lime salts.
Crayfishes have long been
used for food in France
and elsewhere on the con-
tinent of Europe. More
than thirty years ago Huxley
wrote that Paris paid over
$80,000 per year for cray-
fishes. In the United States
they are found in special
markets of the large cities.
Half a million are shipped
annually from the Potomac
River. Oregon ships more than 100,000 pounds. The
demand is increasing, and it will probably pay to have
crayfish farms on which to raise them for market. Land
too wet for agriculture might be so used.
The shrimps seen in New York markets usually come from
the Pacific Coast, and the trade in them is worth many hun-
FiQ. 88. Blind crayfish from Mam-
moth Cave. {From Packard.)
ECONOMIC RELATIONS OF OBGANISM8
349
Fio. 89. A common spider.
Four pairs of legs, while insects
have three. Abdomen much
larger than head-thorax. {From
Parker and Haswell.)
dred thousand dollars annually. Only the abdomens are
commonly sold in the markets, and the bright red color is
due to their having been boiled.
( L) If time allows, some brief labo-
ratory studies of crayfish or lobster
and crabs should be introduced at
this time.
318. Economic Relations of
Spiders and their Allies.* — In
the lessons on animals that affect
human health it is stated that
the fear which most people have
concerning spider bites is un-
founded. Instead of being dan-
gerous to man, the spiders are
useful as destroyers of insects,
which they catch by means of their webs. The silk of the
webs may be made into cloth finer than that from the silk-
worm, but it is much more difficult to rear spiders and collect
the silk from their spinning organs (spinnerets) . The spider's
silk is so deUcate that it has often been used for the " cross-
hairs " which mark the centers of telescopes.
The best known relatives of the spiders are the scorpions.
Concerning their bites, see § 265. Like the spiders, they are
destroyers of insects.
The mites and ticks belong to one of the groups of spider-
like animals. The itch-mite (Fig. 91) burrows in the hu-
man skin, causing itch, and another species causes " mange *'
in the skin of dogs. The ticks live as parasites on skins of
♦ The spiders are interesting animals for observation. For the general
points of structure, see the " Applied Biology." The chapters on Arachnids
in Davenport's '* Elements of Zoology" and Jordan and Heath's ''Animal
Forms" should be read by students. See also " Cornell Leaflets" or Com-
stock's " Nature-Study."
350
INTRODUCTION TO BIOLOGY
various animals. Sheep ticks cause ill health of sheep and
great financial loss to stockmen. Cattle ticks spread the
germs of the destructive disease known as Texas fever. In
Africa there is a human disease known
as " tick-fever.'* It is probable that
other disease germs are spread by
these skin parasites, and hence all
of them which bite man or the useful
birds and mammals should be re-
garded as injurious animals.
319. Usefullnsects. — Honey-bees
and silkworms are the only truly
domesticated insects. A few others
are directly useful for their products,
— " Spanish flies " (used in medi-
cine), cochineal bug (cultivated on
cactus for the dyes cochineal and
carmine which their dried bodies
yield), and the lac-insect (which produces the valuable
shellac used in varnishes). Some natives of
Africa, Australia, and Mexico eat certain in-
sects.
As agents in cross-pollination of flowers, in-
sects as a group are worth more than what
they destroy; but it happens that many of
the very destructive insects do not visit and
pollinate flowers. Many of our most useful
plants depend upon pollination by insects, e.gf.,
clover, alfalfa, fruit trees, and most vegetables.
The Smyrna fig is now successfully cultivated in California
because an insect imported from Algeria pollinates the flowers.
Some insects are vaulable as destroyers of injurious insects.
Numerous insects have their insect enemies, some predatory
Fig. 90. Scorpion. Four
pairs of legs, a pair of
pincers, and one of man-
dibles. Sting at end of
** tail." Four pairs of
breathing pores on larger
part of .the abdomen.
{From Kingaley.)
Fig. 91. Itch-
mite, a skin
parasite.
Note four
I>airs of legs.
ECONOMIC RELATIONS OF ORGANISMS 351
and some parasitic. As an example of predatory insects may
be mentioned the history of the fluted scale-bug which once
threatened to destroy the orange groves of California. The
scale originally came from Australia, and there the ento-
mologists of the United States Department of Agriculture
found a natural enemy in a species of lady-bird beetles.
Some of these beetles were imported to California and in a
few years practically exterminated this species of scale-bug.
Specimens of the beetles sent later to other countries have
been successful in ridding orange and lemon trees of the de-
structive scale-bug. This is one example of the usefulness of
a predatory insect. Many with similar habits are constantly
keeping harmful insects in check.
Parasitic insects are important checks on injurious insects.
A large number of species of insects belonging to the Diptera
and Hymenoptera are parasitic during their larval stage.
The caterpillars of many moths and butterflies are frequently
parasitized by larvae of certain flies. The ichneumons
(Hymenoptera), of which more than ten thousand species
are named, are famous parasites. Some of these insects have
ovipositors three and four inches long and are able to bore
deeply into trees in order to lay their eggs in the larvae of
another insect. " Tomato-worms '' (larvae of hawk-moth)
and similar moth larvae are often seen with their skins covered
with small white cocoons. The parasitic larvae live inside
the moth larvae and crawl out to the surface of the skin when
ready to pupate. The result in most cases is the death of
the parasitized caterpillars.
The above are simply illustrative examples chosen from
the thousands of cases in which insect parasites destroy other
insects. In all such cases where the insect host is harmful
the parasite is beneficial.
Finally, it must be mentioned that insects are iiseful as
352 INTRODUCTION TO BIOLOGY
foods for numerous species of birds. True, it is often stated
that the birds are useful as destroyers of insects; but in-
telligent people are beginning to recognize that many birds
would be useful and well worth supporting for aesthetic
purposes even if there were no injurious insects to be de-
stroyed. Aside, then, from the great outbreaks of certain
insects, a limited number of them are desirable as food for
interesting birds.
Also, it should be mentioned that many fishes eat large
quantities of insects, both adults and larvae. This is why
imitation insects are used as bait by anglers.
320. Injurious Insects. — Those insects which injure or
destroy useful plants and animals, or organic products (e.gf.,
foods and textiles) which are of value to man, or which injure
man himself (as by infecting with malaria), are conveniently
grouped as injurious insects. The truth is that a very large
proportion of insect species are injurious, but usually they
do not attract attention unless they become excessively
numerous, or unless man develops a special interest in an
animal or plant which they injure. For example, probably
more than eight hundred species of insects attack oak trees ;
but it is rare that enough appear on any one tree to do any
noticeable damage, and as long as there are plenty of oak
trees no one cares how many kinds of insects live on them.
A few statistical estimates will give some idea of the damage
which a single species of insect can do. The grasshoppers
(Rocky Mountain locusts) destroyed crops to the value of
$200,000,000 in Iowa, Missouri, Kansas, and Nebraska in
four years, 1874-1877. Enormous damage has been done
by chinch-bug on cereal plants, by Hessian fly on wheat, by
scale-bug on fruit trees,, by gypsy moth on forest and fruit
trees, by cotton-boll weevil, and by numerous others which
do great but less damage than those mentioned. Famous
ECONOMIC RELATIONS OF ORGANISMS 353
entomologists have estimated that insects damage farm
crops in United States annually to the extent of $300,000,000.
Insect damage to valuable forest trees is on good authority
estimated at $100,000,000, yearly. Add to these figures the
enormous loss of animals through disease caused directly or
indirectly by insects ; the destruction of clothing, foods, and
other useful articles ; the value of the working time and ex-
pense of treatment of people who are ill through diseases
caused by insects, and the total annual cost of insect damage
in this country is probably more than the combined cost of
the army and navy and the public-school system. Such gen-
eral estimates suggest the immensity of the problem of deal-
ing with the injurious insects.
And yet such statistics must not be taken as a declaration
of war against insects indiscriminately. They simply mean
that efforts must be made to hold in check the ones which
are noticeably harmful. The ordinary insects which one
meets during a long walk in the country are not likely to
appear in such numbers or to develop such new habits as to
be of special economic interest. Hence there is no reason
why we- should destroy them. On the contrary, this old
world is of greater interest because of their existence. As an
illustration, katydids and crickets do eat some leaves of
grass and other plants, but to many a person who is interested
in natiu'e-study a mid-summer night's chorus by these insects
is worth far more than the trivial damage they do.
The Control of Injurious Insects, — The investigations by
entomologists in the past fifty years have made great prog-
ress towards controlling injurious insects. Knowledge of
habits and life-histories has made it possible to prevent
destruction of crops. Here are a few from hundreds of ex-
amples recorded in the large works on economic entomology :
The discovery that the fruit-moth (codling moth) lays its
2a
864 INTRODUCTION TO BIOLOGY
eggs in the calyx of apple flowers after the petals have fallen
suggested the desirability of spraying the trees with arsenical
poisons before the larvae hatch and biurow into the fruit.
Grasshoppers lay their eggs a few inches below the surface
of the soil, and hence shallow plowing in the autumn will
expose the eggs to the winter storms. The scale-bugs and
plant-Uce live by sucking sap from plants, and hence poisons
like arsenicals cannot reach their stomachs, and the logi-
cal conclusion is that they ought to be sprayed with lime-
sulphur, or petroleum, which kill by contact. These cases
are simply illustrations of the fact that all the satisfactory
methods of dealing with injurious insects are based upon
careful biological study of the species in question.
321. Economic Relations of Starfish. — This animal is a
member of the group of echinoderms, which contains the
starfishes, sea-urchins, crinoids (sea-lilies or feather stars),
and sea-cucumbers. Most of these are of interest only in
zoology, and they are attractive to advanced students be-
cause they are not closely related to any other groups of
animals and have many striking peculiarities. Certain large
sea-cucumbers of the Pacific are dried and form an impor-
tant article of commerce with the Chinese, who consider
them a delicacy. The only other echinoderms of marked
economic interest are certain species of starfishes which feed
on oysters and clams, and are often so numerous as to cause
great loss to owners of oyster-beds. Since a starfish has
no jaws and an extremely small mouth, it is evident that
only by some unusual method of feeding could it eat an
oyster. This is accomplished as follows: The starfish
stomach is a large, thin-walled sac which can be everted
through the small mouth, much as* one might turn a glove-
finger inside out. A starfish fastens its suckers on an oyster,
and then the stomach covers the edges of the oyster's shell
ECONOMIC RELATIONS OF ORGANISMS 355
with the result that the circulation of water inside the oyster's
shell is stopped and the animal is killed by suffocatioji. The
oyster shell then gapes open, the starfish's stomach pours in
its digestive secretion, the tissues of the oyster are dissolved
(digested) while in its own shell, and then the digested sub-
stances are absorbed by the starfish. Finally, the starfish
withdraws its stomach into its own body and leaves the empty
shell of the oyster.
Against such a remarkable enemy an oyster or clam is
completely helpless, for the hard shell which protects against
enemies that feed with jaws is of little avail against the
starfishes, which are the only animals able to evert their
stomachs for the purpose of surrounding and digesting food
too large to be taken into the mouth.
Owners of oyster-beds now make systematic efforts to
destroy starfishes; and steam dredges are used to bring
both oysters and starfishes to the surfaces. Formerly, the
oyster fishermen used to break the arms from starfishes and
throw them back into the sea ; but the discovery that star-
fishes have the ability to regrow or regeneratCy and that each
of many pieces may soon form a perfect starfish, showed that
breaking them into pieces simply multiplied them. Now,
when starfishes are caught by oyster-dredges, they are killed
instantly in boiling water, or they are left on dry land where
they die quickly.
(D) Exhibit specimens of various echinoderms for the sake of
general acquaintance. Also use illustrations in books of zoology.
322. Economic Relations of MoUusks. — The group of
the shell-animals or moUusks (MoUuska) includes such
familiar forms as clams, oysters, snails, garden slugs, " sea-
shells," and the rarer chitons, squid, nautilus, octopus, and
others. If time allows for careful study of some of these,
356
INTRODUCTION TO BIOLOGY
Fig. 92. Diagram of a land-
snail (Helix). «, shell ; a,
anus; h, breathing pore
opening into the "lung";
r, opening of reproductive
organs ; t, four tentacles;
e, eyes at end of longer ten-
tacles. {From Parker and
HaaweU.)
see textbooks of elementary zoology or §§ 336-342 in the
" Applied Biology."
The value of the oyster industry in America and in
Europe is enormous. Studies by
zoologists connected with govern-
ment laboratories have vastly im-
proved methods of propagating,
and have made it possible to raise
oysters where they do not naturally
grow. Oyster-beds are now regu-
larly leased by states to oyster-
men, and oysters are artificially
" planted." In many places it
is necessary to rake the sea-bot-
tom with steam dredges annually
in order to bring the oysters to the siu'face and free them
from the destructive starfishes. The egg-laying months are
May to August, and the
popular saying that oysters
are not edible except in
the months with the letter
" r " in their names, i.e.,
September to April inclu-
sive, is connected with the
fact that in the months
without " r " (the summer
months) the animals are
likely to be filled with eggs.
The oysters are the most
valuable moUusks, and the
business of raising oysters
is worth millions of dollars a year. From Chesapeake Bay
alone more than twenty-five million bushels of oysters are
marketed annually.
Fig. 93. A marine clam. All below
the line x— j/ is usually imbedded in the
sand, h, hinge of the shell; u, umbo;
I, parallel lines of growth of the shell;
m, edge of mantle; /, foot; e, i, exhalant
and inhalant siphons, the arrows in-
dicating the direction of currents of
water. (From Verrill.)
ECONOMIC RELATIONS OF ORGANISMS
357
The marine clams (Fig. 93) and scallops are of great
importance as food, and attempts are now being made to
cultivate them. Unless this becomes commercially success-
ful, the natural supply will soon be exhausted. Investiga-
tions are being made under the auspices of the United States
Bureau of Fisheries and of certain state experiment stations ;
and these scientific institutions deserve the necessary finan-
cial support by legislative bodies.
Certain large land-snails (Fig. 92) have long been es-
teemed as delicacies. There were snail-gardens in Roman
times, but now the snails are widespread pests in vineyards
and gardens of Europe. Many are collected and shipped to
American dealers in food delicacies.
Preparations from snails were once used for coughs, con-
siunption, malaria, asthma, dropsy, and
almost all other diseases. In some rural
regions of England people still believe that
snails are of medicinal value.
Niunerous marine snails (gasteropods)
are used as food in various parts of the
world. Squids (Fig. 94) and cuttle-fishes
are eaten by poor people in some countries.
Sepia or India ink has been obtained
from cuttle-fishes. The famous Tyrian
purple, once used for coloring royal robes,
came from another moUusk.
Shells of moUusks are of great ornamen-
tal value. They have long been sought by
conchologists (collectors of shells), and more
than $100 has been paid for a single rare
specimen.
Natives of the islands in the Pacific Ocean use shells for a
great variety of purposes, ornamental and useful. In some
Fig. 94. Male
squid, a, arms
— two long ones
in male; e, eye; i,
opening to siphon;
/, fin. {After Ver-
rill.)
358 INTRODUCTION TO BIOLOGY
barbaric tribes of Central Africa shells still pass as money,
and the ^* money cowry " is the standard currency. The
North American Indians once cut cylindrical beads from the
purple spots on shells of the hard-shelled clam (Venus),
and this was at least one kind of " wampum " mentioned in
early colonial history of the United States.
Civilized men make extensive use of shells. Some are
used for inlaying and for other ornamental work. Vast
quantities are used for pearl buttons. Pearls of great value
are each year collected from clams of various species. In
North America many pearls
are found in the rivers of the
Mississippi system and in
, . , Lower California.
FiQ. 95. Garden slug (Limax). ^ , , ,^. ^-v
t, tentacles; m, mantle covering Garden slugS (*lg. 95) are
rudimentary sheU; b, breathing like land-Snails with degener-
^ ^' * ate shells. They are some-
times so abundant in gardens that their feeding causes great
damage.
(Lot D) Specimens of the various types of mollusks should be
examined at least for the purpose of obtaining a general acquaint-
ance; and if time permits, it is advisable to make a laboratory
study of a clam and snail according to || 337 and 339 in the
" Applied Biology."
323. Economic Value of Fishes. — A few examples of
fishes which are important in the human food-supply will
suggest the enormous total value of this group of animals.
External Structure of a Fish. — (L) Any available fish from the
market should be examined. Note the two pairs of lateral fins;
and the dorsal and ventral fins on the median line. Compare
several species of fishes, or pictures of them, as to the arrangement
of the fins ; and especially note that the posterior pair of fins is in
some fishes near the anterior pair. Note arrangement of scales.
ECONOMIC RELATIONS OF ORGANISMS 359
Examine the mouth and gills.
Observe (1) the movements of the body and fins, and (2) the
mouth and gills of living fishes in an aquarium.
Salmon worth more than $13,000,000 are annually caught
on the Pacific Coast of North America, nearly one-third of
these from Puget Sound and Columbia River. The average
weight of a full-grown salmon of the Columbia River species
is over twenty pounds, and individuals have weighed 100
pounds.
Herrings are probably the most valuable food fishes in
the world. Huxley estimated that three billion, each aver-
aging half a pound in weight, are caught in the North Sea
and Atlantic annually ; and this is now too low an estimate.
They swim in enormous groups or ' • shoals " which some-
times extend over half a dozen square miles. '* Sardines "
from Maine are simply small herrings, but the true European
sardines belong to another species.
The codfish is one of the most important North American
fishes. About 7000 men are engaged in the fishery, and the
annual catch is in some years near 100,000,000 pounds and
worth to the fishermen about $2,000,000. , The cods live in
deep water (20 to 100 fathoms), and ar.e captured only with
baited hooks and lines. A cod over six feet long and weigh-
ing over 200 pounds was once taken; but from 12 to 40
pounds are the usual weights.
Next to the Columbia salmon and the cod, the shad is the
most important fish caught in the waters of North America.
It is captured in the spring when it ascends rivers to spawn.
The annual catch is about 14,000,000 fish, weight 50,000,000
pounds, and worth more than $1,600,000. The fact that the
shad is taken only at the spawning season would long ago
have made the fisheries unprofitable if the United States
Fish Conmiission had not engaged in artificial propagation.
360 INTRODUCTION TO BIOLOGY
More than 200,000,000 young shaxi are annually *' planted ''
in the rivers of the Atlantic Coast. They have been intro-
duced since 1871 on the Pacific Coast, where they do not
naturally occur; and have now become abundant in the
markets of the west-coast cities. No better proof could be
desired as to the value of the work of the government in
artificial propagation of fishes. And this is only one of many
fishes which has been widely distributed and made more
abundant as the result of science applied by the experts on
fish culture.
The three most valuable North American fishes have been
selected to illustrate this discussion of economic value ; but
there are many others worth tens of thousands of dollars
annually. The fact is that we have scarcely begun to learn
the real value of fishes as a source of meat food. There are
hundreds of rivers, lakes, and ponds which might be stocked
with fish of selected species and made to produce an abun-
dance of good food, the fish, moreover, destrojdng numerous
larvae of the dreaded mosquitoes. It is certain that the
successful methods of artificial hatching and distribution
which have been discovered by experts in the government
service will ultimately make many useless bodies of water
profitable to their owners.
The student who is interested in fishes should refer to Jordan and
Evermann's "American Food and Game Fishes."
324. Economic Relations of Amphibians. — The class
Amphibia includes the frogs, toads, and the tailed forms
which are popularly called newts, salamanders, mud-puppies
(Necturus), water-dogs, and mud-eels. Some of these are
often mistaken for lizards ; but lizards (e.g., chameleons) are
reptiles with scaly skin, while the common amphibians have
smooth skin like that of the frog. Some of the tailed am-
ECONOMIC RELATIONS OF ORGANISMS 361
phibians have gills in the adult state, and live in water.
Those without gills respire by means of the skin when in
water, and by lungs and moist skin when on land. It is
well known that frogs' legs are eaten; and it has proved
profitable to catch frogs for the market. In fact, the annual
hunting of nearly $50,000 worth of wild frogs will soon exter-
minate them unless " frog-farms " are more extensively de-
veloped in the near future. But the chief value of our
common amphibians is that they destroy thousands of
insects. The common toad is a valuable inhabitant of
gardens. See the bulletin on the American toad which is
published (free) by the United States Department of Agri-
culture, and also see a chapter in Hodge's " Nature Study and
Life.'' We ought to have laws protecting toads as well as
birds, but until we get such laws we must depend upon the
good sense and fair play of intelligent people who are in-
formed concerning the value of toads. Frogs are likewise
useful as destroyers of insects, but have not attracted so
much attention as the toad, which is able to go far from
bodies of water and can be kept in gardens.
The newts and other species of tailed amphibia which
spend much time on land are probably useful destroyers of
insects.
Myths concerning Toads, — The skin secretion in some
toads is disagreeable or even poisonous to their enemies,
but produces no serious effect on human skin. Warts on
small boys' hands are not caused by handling toads. The
truth is that warts may be caused by slight scratches which
allow dirt to get into the skin. Moreover, killing a toad
will not ^^ cause your father's cow to give bloody milk," for
a moment's serious thought shows us the absurdity of the
folklore that there is such a relation between cows and toads.
Both the wart and the milk legends are absolutely unscien-
362 INTBODUCTION TO BIOLOGY
tific. There are numerous other absurd beliefs concerning
common animals, and one who has studied biology will
always demand proof or the authority of some modern
scientific book before accepting them.
326. Economic Relations of Reptiles. — There are four
types of reptiles: lizards, turtles, snakes, and alligators.
Most lizards are harmless, and may be useful as destroyers
of insects. Iguanas and other large species are hunted for
their flesh. Brilliantly colored lizards are often kept as
pets ; they should be fed insects, and not starved on sugar
and water. (Why?)
Many species of turtle are valuable as human food. Ter-
rapin turtles are now so very high in price that " terrapin-
farms " are profitable. Beautiful tortoise shells, which are
plates or scales from the back of a tortoise, are made into
combs and ornamental articles.
Poisonous snakes have been mentioned in § 266. Most
American species are not directly harmful to man, but some
are really useful in that they destroy such rodents as rats
and mice. However, those that eat rodents are also fond of
young birds and eggs, and are indirectly injurious because
most birds of the kinds killed by the snakes are useful and
should be protected. Some of the non-poisonous snakes
prefer frogs and toads, which are useful as destroyers of in-
sects. There is so much variation in the food of common
snakes that one should consult Ditmar's '* Reptile Book "
before deciding that any given species is indirectly harmful
because it kills useful animals.
Alligators are valuable for their hides. In fifteen years
(1880-1895) 2,500,000 were killed in Florida. The eggs are
edible, and many people hunt eggs during the proper season.
Curio dealers sell to tourists large numbers of young alli-
gators, which almost always die of starvation. The species
ECONOMIC RELATIONS OF ORGANISMS 363
will soon be extinct except in the most inaccessible
swamps.
326. Economic Relations of Birds. — In several ways
birds are of practical or economic interest to man. Probably
most important is the use of certain species for food. Some
of these (ducks, geese, chickens, pigeons, turkeys, Guinea
fowls, and pheasants) have domesticated varieties or breeds
which for meat and eggs are more valuable than the original
wild species. Numerous wild species of ducks, geese, grouse,
pheasants, quails, turkey, and many so-called " game birds ''
are useful as food, but most of them have been hunted so
persistently that they are no longer abundant enough to
furnish a very important part of civilized man's food-supply.
Feathers of many birds are used for ornamental purposes,
and those of others for pillows. Ducks, geese, and ostriches
are the most important of the domesticated birds which are
kept for the feathers that are plucked before the birds
are ready to shed or molt them naturally. The feathers of
chickens and some birds are saved when the birds are dressed
for the market. The most beautiful of ornamental feathers
are obtained from wild birds such as certain herons (egrets),
paradise birds, parrots, etc., which are usually killed in order
to get the feathers. The milliners'* demand for these is
rapidly leading to extinction of many species of beautiful
birds. In the case of the snowy heron or egret, from which
aigrettes are obtained, the birds are always killed when they
have young nestlings and these inevitably starve to death.
This is the reason why sensitive women who know the story
of the egret will not wear aigrettes and why the societies
for the protection of wild birds have been so active in trying
to stop the sale of the plumes. In the cases of many other
beautiful feathers used for millinery, most awful cruelties
are practiced by the hunters who collect them for market.
864 INTRODUCTION TO BIOLOGY
The Audubon Societies, which are interested in bird study
and protection, have for many years been working for legis-
lation which will stop the commercial handling of feathers
of many kinds of wild birds, for this is the only practicable
way of discouraging the hunters.
Besides the direct usefulness of birds for food and feathers,
many of the wild birds are of value because they eat injurious
insects and weed seeds. All species of sparrows and finches,
juncos, mourning doves, quails, pigeons, and grouse are some
of the birds known to feed largely on seeds of useless plants
and weeds. However, there are no exclusively vegetarian
birds. A large proportion of the food of most common
birds consists of eggs, larvae, or adult insects, many of
which are injurious. Thus indirectly such birds are useful
to man. Flycatchers and swallows are the best insect
hunters; but woodpeckers, cat-birds, robins, blackbirds,
crows, blue-jays, cuckoos, some hawks, warblers, chickadees,
various thrushes, some owls, and many other birds are credited
with making at least part of their meals on injurious insects-
Many of the hawks and owls are beneficial in that they
prefer mice and other rodents. Even a few field-mice cause
appreciable damage to meadows and grain fields; and in
many parts of Europe there are occasional plagues of the mice.
Certain birds destroy seeds, fruits, and useful plants.
Some of the same birds are also destroyers of injurious in-
sects and weed seeds. The robin is an example ; sometimes
it eats cherries and other small fruits, but most of the year
it must get other kinds of food, and on the whole is a useful
bird. Any birds that eat large seeds may destroy ripe
grain in the fields, and sometimes do considerable damage.
However, it is probable that the same kinds of birds destroy
enough insects at other times to compefisate for the damage.
An exception is the case of the rice fields which are visited
ECONOMIC RELATIONS OF ORGANISMS 365
by countless thousands of migrating blackbirds and rice-
birds (bobolinks), and these do serious damage without
being at any time of appreciable benefit to the rice planters.
The birds of prey are charged with destroying* useful
birds; e.g., certain hawks and owls occasionally destroy
poultry. However, most of these as destroyers of mice and
insects more than pay for the damage, and they should not
be killed indiscriminately. Cooper's hawk, the sharp-shinned
hawk, the goshawk, and the great horned owl are the only
ones whose attacks on useful birds may justify kilUng them,
especially in some places where they are abundant.
There is much doubt as to whether certain kinds of birds
should be protected, for the evidence for and against them
is nearly equal. Examples are the crows, blackbirds, blue-
jays, bobolinks (rice-birds), English sparrows, the above-
named birds of prey, and many others. Local overabundance
often makes it desirable to destroy some of these birds in
order to reduce the numbers ; but they should not be exter-
minated.
The relations of birds to man should not be considered
entirely from the economic point of view presented in the
foregoing paragraphs, for it is probable that there are more
people who enjoy the birds as beautiful and interesting ani-
mals than who have a purely economic interest in them.
The great majority of birds deserve protection because of the
widespread interest in them as well as because they are
useful to man.
References. — Farmers' Bulletin 54 and several other pamphlets
on the economic relations of birds are distributed to applicants by
the United States Department of Agriculture. Weed and Dear-
bom's " Birds in their Relation to Man " is a very readable sum-
mary of the best literature on the subject. The most popular
magazine devoted to birds is " Bird-Lore " ($1 per year).
366 INTRODUCTION TO BIOLOGY
327. Economic Relations of Mammals. — No other class
of animals approaches that of the mammals in economic
importance. The truth of this statement will be obvious
to any one who considers the vast monetary valu« of the
conmion domesticated mammals — horses, cattle, sheep, pigs,
goats, and dogs. Also, in some countries, camels, elephants,
llamas, and reindeer are important domesticated mammals.
The domesticated species are useful in (1) the human
food-supply, and (2) as beasts of burden.
Many wild manmials are also useful to man. The most
valuable of these are the fur-bearers (chiefly carnivores, such
as seals, foxes, mink, bears). Whales have long been hunted
for the whale-bone obtained from their jaws and the sperm-
oil from their " blubber.'' A large whale of one species may
yield over $10,000 worth of whale-bone and three hundred
barrels of oil. Elephants and walruses have been ruthlessly
hunted for the ivory of their tusks. Sea-cows are hunted
for their flesh, oil, and hides. Beavers have been nearly
exterminated because of their valuable skins.
Modern methods have made it possible to utilize every
particle of manmials slaughtered for human food. In addi-
tion to the meat obtained, the poorer qualities of fat are
made into soap ; the horns into combs ; the hoofs into glue ;
the best hair into packing for many purposes, while the poorer
grade is used in plastering walls of buildings ; gelatin is made
from tendons; leather from the dermis of the skin; lean
scraps and blood are dried to make foods for poultry and
other animals; bones are made into hundreds of useful
articles and the best small pieces are ground into bone-meal
for feeding poultry; and any particles of bone, blood, or
flesh, not usable in other ways, are dried and pulverized to
make commercial fertilizers for agricultural use.
The dog was the first domesticated animal, and it is in-
ECONOMIC RELATIONS OF ORGANISMS 367
teresting to note that primitive dogs were probably chiefly
kept as pets and companions, just as many of our modern
dogs are to-day. The development of such uses as hunting,
guarding flocks of sheep, and drawing sledges and carts seems
to have come after the dog's masters began to emerge from
the lowest barbarism.
Among very injurious manmials are numerous species of
rodents (e.g., rats, mice, gophers, prairie-dogs, certain
squirrels, rabbits in Australia) ; some carnivores (e.g., the
dangerous cat-like species, the weasel-like forms, the bears
and wolves) ; and fruit-bats.
Some mammals which are insect-eaters are indirectly
beneficial. Many bats, ant-eaters, and the moles are ex-
amples ; but owing to their subterranean habits the moles do
much damage to roots among which they burrow in search
of larvae of insects.
References. — Concerning the injurious mammals (rodents,
wolves, etc.) there are many pamphlets issued by the United States
Department of Agriculture. Two of the most interesting books
dealing with domesticated mammals are Shaler's " Domesticated
Animals " and Wood's " Dominion of Man."
CHAPTER XII
THE REPRODUCTION OF ORGANISMS
328. Individual Life Limited. — We have already noted
that all individual animals and plants grow old and die,
and that reproduction or generation of new individuals
(offspring) is necessary for the perpetuation of any species.
For example, individual horses do not live fifty years, and
it is evident that unless new individuals are reproduced by
the present horses, the horse species will become extinct
within fifty years. Reproduction, then, is a necessary
process of developing new individual organisms for the per-
petuation of species.
Within the limits set for this book there is space for de-
scribing the reproductive processes of only a few selected
animals and plants. It will be most interesting and instruc-
tive to begin with some of the simplest (the one-celled) ani-
mals and plants. These probably existed on the earth before
the higher organisms appeared, and their reproduction rep-
resents the simplest methods by which species can be con-
tinued by production of new individuals to take the places
of their parents.
REPRODUCTION OF THE SIMPLEST ORGANISMS
329. Reproduction of One-celled Animals. — Amoeba,
Paramecium, and all the other one-celled animals reproduce
by automatic division of their bodies into two equal parts.
These are young individuals able to take food and grow,
368
BEPRODUCTION OF 0B0ANISM8
869
Fig. 96. Division or re-
production of a Styloni-
chia similar to that of
Paramecium, n, nucleus;
V, vacuole. {From Hat-
schek.)
and when of full size they in turn divide. Examine Figs.
96 and 97. When food is abundant the paramecia will
divide about three times in forty- a. r
eight hours, that is, one individual
will divide into twO, these into four,
and these into eight within two days.
It is interesting to note that in
this simple form of reproduction the
parent animal loses its individuality
when it reproduces. Contrast this
with higher forms, as the frog, which
produce young, while the parent con-
tinues to live until it grows old and
dies. It is evident that, barring
accidents and disease, there is no
chance for a Paramecium or an amceba
to grow old and die; for when it
reaches full size, it divides into two young animals, which
in turn take food, form new protoplasm,
grow to the full size, and divide.
Conjugation, — Under certain condi-
tions, paramecia reach a state in which
they are unable to continue to divide.
Two such individuals come into contact,
and through their delicate cell-walls
some of the nucleus of each one passes
A B . over to join the nucleus of the other.
Fig. 97. Division of a Then the two animals swim away inde-
m^^c!ue).^*^S?i^e pendently, each soon divides, and their
nucleus; », vacuole, offspring may continue to divide for
{From Hatschek.) ^ j^^^ ^^^j^^ ^j generations before this
process, known as conjugation, again takes place. Other
kinds of one-celled animals conjugate.
2b
370
INTRODUCTION TO BIOLOGY
330. Reproduction of a One-celled Plant. — A green
coating or stain which is often seen on the shaded (usually
north) side of tree trunks, unpainted walls, etc., will be found
upon microscopic examination
to be composed of masses of
one-celled plants (Pleurococ-
cus), often arranged in groups
of two, four, or more cells (Fig.
98) . This grouping is the result
of repeated cell-division. Each
cell has protoplasm, nucleus,
chlorophyll-bodies, and cell-
wall. Having chlorophyll,
they are able to make carbo-
hydrates from carbon dioxide and water. Particles of dust
lodged on the wood on which they grow probably supply
the necessary materials which higher plants absorb from
the soil. Abundance of water favors growth, and they
appear a brighter green during rainy weather.
Bacteria (§ 239) and yeast (§ 280) are also good examples
of the simplest method of plant reproduction, by division.
Fio. 98. Division of a one-celled
plant (a) into two cells (6), three
cells (c), four cells (d). {From
Sedgwick and Wilaon.)
THE SIMPLEST METHOD OF REPRODUCTION
331. Asexual Reproduction. — The one-celled animals
and plants illustrate a type of reproduction in which sex is
not involved. The individuals are not male or female, but
they are all alike and capable of reproducing by division.
There are no eggs or ovules such as we know form the new
individuals of higher animals and plants. Such reproduction
in which there are no eggs but in which the new individuals
are formed by division of the parent individuals is known as
asexual reproduction (meaning reproduction without sex),
REPRODUCTION OF ORGANISMS 371
while such development as that of insects and fishes from
eggs and of seed-plants from ovules of flowers is sexiud
reproduction.
In most cases of sexual reproduction of animals and plants
there are two parents, the female, whose ovaries produce ova
or egg-cells, and the male, whose spermaries produce sperms
or sperm-cells ; and an egg-cell does not develop until it has
been entered and fertilized by a sperm-cell. This entrance
into an egg of a sperm-cell is called fertilization. There are
some species of insects and plants able tb produce egg-cells
capable of developing without being fertilized.
In addition to the reproduction by division of the one-
celled organisms, there are many examples of asexual re-
production among the many-celled animals and plants.
332. Asexual Reproduction in Animals. — The Hydra
(§ 116) reproduces asexually as long as it has abundant food
and other favorable conditions of life. A small elevation
(called bud) appears on the side of a hydra and within a few
days grows to form a small hydra (Fig. 53). This soon
becomes detached, and lives independently. If food is
abundant and a hydra is growing rapidly, it may have two or
three buds developing at the same time and the young ani-
mals formed may soon reproduce by budding.
Under certain unfavorable conditions, such as lack of
food, stagnation of water, etc., hydras may cease growing
and budding and develop reproductive organs (ovaries and
spermaries), which form egg-cells and sperm-cells. When
the spermaries of a hydra are mature, the sperm-cells escape
and swim freely in the water. A single ovum or egg-cell
develops in an ovary, and is fertilized by the entrance
of a single sperm-cell (§ 348). The fertilized egg-cell
forms an embryo, and becomes surrounded by a hard
shell or cyst. The protected embryo falls to the bottom
372
INTRODUCTION TO BIOLOGY
of the pond,- and it may remain there for some time. Dry-
dust scraped from bottoms of ponds during mid-summer
drought may contain embryos which will develop into hydras
soon after being placed in water.
Hydras have another reproductive process which occurs
only when by some accident an individual is cut into two or
more pieces. The remarkable fact is that within a few days
each piece will grow into a perfect hydra. This is an ex-
ample of regeneration. Many other lower animals have the
same power of forming a perfect body from a part, and some
animals as complex as frogs can regenerate small parts, such as
toes, if they happen to be destroyed. It should be noted that
FiQ. 99. Hydroids growing on a mussel shell. A tree-like group, such as
d, is formed by repeated budding (compare o, 6, c,d). A single, free-swim-
ming larva, from one egg-cell, attached to the shell and formed the first
hydroid, from which all these originated by budding. (After Schulze.)
REPRODUCTION OF ORGANISMS 373
regeneration is not a regular reproductive process in hydras,
but provides against accidents.
The marine animals known as hydroids, which are near
relatives of Hydra, form buds that do not become detached,
but remain and grow into branches similar to those of a
tree (Fig. 99). In this way a single hydroid developed from
an egg may by repeated budding grow into a tree-like colony
in which there is an hydroid individual at the end of each
branch (see Fig. 99) . Some of these individuals form medusae
or jellyfishes, which become detached, float in the water, and
form eggs and sperms. Each fertilized egg develops into a
young hydroid which attaches itself to a rock and by re-
peated budding forms a new hydroid colony like that repre-
sented in Fig. 99.
Most worms reproduce by fertilized eggs (i.e., sexual
reproduction), but some have also asexual reproduction.
The marine worm represented in Fig.
100 is able to divide off new worms at
its posterior end, and sometimes sev-
eral such divisions may be occurring
at the same time so as to form a chain
of small worms. This chain finally
breaks and the individuals become in-
dependent.
The complex animals such as the
crustaceans, insects, spiders, snails, ^V^^meZ^'Z^
and vertebrates do not reproduce duces by division. {After
asexually; but only from eggs which, Milne-Edwards.)
with few exceptions, must be. fertilized in order to
develop.
333. Regeneration. — Many animals are able to regrow
or regenerate parts which have been cut off. If an earth-
worm is divided by a gardener's spade, the two parts may
874 INTRODUCTION TO BIOLOGY
grow into perfect worms. A starfish may be broken into
several pieces and each will regenerate and form a perfect
starfish. A hydra may be cut into many pieces and each
will form a perfect hydra of small size within a few days.
Such power of regeneration is common in the groups to which
amoeba, hydra, flat-worm, earthworm, and starfish belong.
Many animals of the higher groups are able to regenerate
certain organs, such as legs ; but cannot form two or more
complete individuals from the pieces of one animal.
Regeneration is not a regular method of reproduction,
but in the case of animals like hydras, earthworms, and star-
fishes, it may result in multiplication of individuals whenever
any accident causes division of one of these animals.
334. Asexual Reproduction in Plants. — Numerous plants
reproduce without flowers, egg-cells, or other structures con-
nected with sexual reproduction. Some of the best-known
examples are among the flowering plants. While these
naturally reproduce from seeds, which are formed by proc-
esses of sexual reproduction in flowers, many of them may
reproduce from roots, stems, and even leaves. The following
are common examples :
Strawberry plants are commonly obtained from branches
(called runners) which creep over the ground and take root
at the joints or nodes, and form new plants at these points.
In this way one plant has formed more than fifty new plants
in a single summer. Black raspberry bushes and grape-
vines bend over, form roots, and develop new plants where
they touch the ground. Red raspberry bushes multiply by
forming young plants from underground branches resembling
roots. New plants start from roots of osage-orange and some
other trees. Some flowering plants may reproduce from
leaves ; the best example is the '* sprout-leaf " or bryo-
phyllum, a garden plant whose leaves fall to the ground,
REPRODUCTION OF ORGANISMS 376
form roots and shoots at notches on the edges of the leaves,
and thus each leaf may form a number of new plants. Some
begonia leaves will propagate in the same way. Still other
flowering plants propagate from bulbs (which are short
stems with leaves). This is well illustrated by certain kinds
or varieties of onions. The common onions (** seed onions ")
are grown from seed in one season, or else the seeds are started
one year and form small onions (" sets '') which are planted
the next spring. In this case one seed forms one set, and one
set forms one onion bulb, which when mature forms flowers
and seeds ; hence multiplication can be only by seeds. Some
other kinds of onion bulbs behave differently, in that the bulb
which is planted divides (asexual reproduction) into a group
of little bulbs (" bulbels ")• The multiplier or potato onion
does this, and garlic is similar. Each of the little bulbels
may be planted and will soon grow to full size, and then
begin to form more bulbels. Multiplier onions occasionally
produce seed, but gardeners commonly use the " bulbels " for
planting. A third kind of onions forms 'Hop onions" or
" bulblets " on the flower-stalks, sometimes mixed with some
flowers. These small bulblets may be planted next season
and soon grow to usable size, and if left in the ground they
will send up flower-stalks which produce bulblets. The so-
called " Egyptian onions " and " tree onions " are top-onions.
In the wild garlic some of the flowers are frequently replaced
by bulblets. The above ways of growing different varieties
of onions illustrate the methods of propagating many bulb-
producing plants. Hyacinth, narcissus or daffodil, many
lilies, crocus, and tulip are examples of flowering plants which
are usually grown from bulbs and rarely from seed.
Without the aid of man many flowering plants may propa-
gate asexually from roots, stems, leaves, and bulbs. By
man's help many of them which do not naturally propagate
876 INTRODUCTION TO BIOLOGY
without seeds can be grown from parts of the plants. For ex-
ample, cuttings or slips may be cut from stems of almost any
plant and if properly set in
moist soil will form roots and
develop into new complete
plants. This is the familiar
method of starting gerani-
ums, coleus, and other house-
plants ; and dozens of shrubs
(e.g., currant), vines (e.g.,
grape), and trees (e.g., wil-
Fia. 101. A rose cutting set in soil low) are commonly started
W. i.Fr^ Cornea Leafieu) from cuttings of stems.
Still another artificial method of propagation from cut-
tings is grafting and budding. The details of this method
are explained in § 301, but essentially it consists in taking a
small piece of stem with a bud from one plant and attaching
it to root or stem of another plant, on which the transplanted
bud grows and forms all the stem above the point where the
bud was attached. This is the method for propagating
standard varieties of apples, pears, plums, cherries, peaches,
and many shrubs and trees.
All such propagation of plants from parts or pieces of
full-grown plants without seeds is asexual reproduction. In
some cultivated plants reproducing by seeds has been sup-
planted largely by asexual reproduction. This has two ad-
vantages : (1) the ease of starting some plants without seeds,
and (2) many cultivated varieties will always produce their
own variety from parts of themselves, but not from seeds.
For example, the seeds from a red apple may produce trees
which bear apples that are green-colored and entirely different
in flavor, size, etc. The only way to propagate with cer-
tainty the desired variety is to take a cutting from a branch
REPRODUCTION OF ORGANISMS 377
of the apple tree known to bear the kind of fruit wanted and
graft it on a tree started from seed. The runners of a straw-
berry plant will form new plants producing the same quality
of berries, but the seeds rarely do so. The tubers (thickened
branches of the stem) of potatoes will produce the same
varieties, but the seeds do not do so. Moreover, in the case
of seedless fruits (e.g., navel oranges and seedless apples)
the only possible propagation is by cuttings, buds, or grafts.
The millions of seedless orange trees in California have de-
scended from a single tree which grew from seed but could
not itself produce seed. Without man's help such a variety
could not have multiplied.
REPRODUCTION BY MEANS OF SEEDS
The usual method of reproduction in the flowering plants
is by means of seeds which are produced in flowers. Hence,
these plants are often called seed-plants, the botanical term
for which is Spermaphytes. They are designated in some
books by the name of phanerogams. In order to under-
stand the parts which flowers and seeds play in the reproduc-
tion of this highest group of plants, it is necessary to study
some flowers, the fruits which develop from the flowers, and
the structure and germination of the seeds in the fruits.
For this purpose the bean flowers, pods (fruit), and seeds are
excellent.
335. Simple Flowers. — In case any students in the class have
not previously learned the parts of any simple flower, they should
study one, identifying: (1) The calyx, composed of sepals;
(2) corolla, composed of petals; (3) stamens, composed of the stalk
or filament and the antfier, hearing minute grains of pollen; (4) in
the center of the flowers the pistil, composed of the ovary, and the
style, which extends upward and has the stigma at its end. Flowers
which have separate petals (scillas, tulips, sedum, lilies, etc.) are
good for such preliminary study of the parts of simple flowers.
378
INTRODUCTION TO BIOLOGY
336. Study of Bean Flower. — (L) Bean plants six to ten weeks
old will fumish all the stages needed for this study. Pea flowers may
be used. Either bean or pea flowers may be preserved in one per
cent formalin solution.
Note how the flower-stalks (pedicels) are attached at the nodes
of the stem and often in the axils of the leaf ; that is, in the angle
between leaf and stem. Identify : (1) two green leaf-like structures
(bracts) at the base of the flower. On very
young flower bud on the same plant these
bracts may be seen inclosing the flower.
(2) Between the bracts and the corolla is the
calyx, composed of five sepals united into a
cup. This can be seen best in an old flower
from which the corolla is ready to fall. (3)
The petals (white, pink, or red) of the coroUa,
five in number and unequal in size, are ar-
ranged as in the diagram in Fig. 102. The
three largest petals are so prominent that at
first sight the flower appears to have only
three ; but two smaller petals are united and
coiled so as to lie between the three largest
petals. The largest petal of a flower like that
of the bean plant is caUed the *' standard " ;
the two petals at the sides of the flower are
; and the united and twisted petals form the
' keel," which lies between the " wings." In a bud just about to
open note how the largest petal (" standard ") incloses the others.
Inside the coil of petals (" keel ") are ten stamens and the pistil.
Look at a flower in its natural position on a plant and note that the
two side petals (the '* wings ") are the only ones on which a bee
could easily alight. If you have opportunity, watch bees visiting
bean flowers in the garden. Hold a flower by its stalk in the natural
position, and with a pencil press downward on the two side petals,
and carefully watch the end of the coiled petals (" keel ") as you
press. Note how the stigma and part of the style appear when
the side petals are pressed, and disappear within the coiled petals
when released. Imagine how a bee could cause the same exposure
of the pistil when alighting on these petals. Hold these side petals
down so as to keep the style protruded and with a hand-lens ex-
amine the stigma, and also notice rows of hairs along the style.
FiQ. 102. Pea flower
8, *' standard " ; to,
"wings"; A;, "keel."
{From Gray.)
called the " wings '
REPRODUCTION OF ORGANISMS 379
Now, carefully uncoil the twisted petals and note how the style
and the ten stamens are inclosed by the coiled petals. The fila-
ments of nine of the stamens are joined together at their base. The
stamens are firmly fastened in place, but the style is not attached to
the spiral tube formed by the twisted petals. The stigma does not
touch the anthers, but the hairs just below the stigma brush over
the anthers, and some pollen-grains cling to them.
The meaning of the structure of the bean flower is this :
When a bee alights on the petals, the stigma and the upper
part of the style are exposed and pollen is brushed on the bee^s
body by the hairs on the style. Then the bee goes to another
flower, and when its style touches the bee's body, the stigma
will touch some pollen from the first flower ; and at the same
time the hairs of the style will brush on to the bee some
pollen from the second flower. And so, as the bee goes from
flower to flower, it will brush pollen-dust on stigmas and get
pollen-dust brushed out from the anthers by the hairs on the
styles below the stigmas. There is very little chance that
the stigma of a flower will get pollen-dust from anthers in
the same flower. This may sometimes happen when a bee
leaves a flower and at once goes back to it, carrying pollen-
dust received on its first visit.
It seems probable from the arrangement of the bean
flower that insect visits are necessary to distribute the pollen-
dust of this kind of flower. Darwin, the famous English
biologist, made many experiments by keeping beans covered
with netting so that insects could not reach the flowers, and
the result was that seeds rarely formed. Pea flowers are
very similar to those of the bean, but the stigma is so near the
anthers that it often gets pollen-dust before the flower is
visited by insects. Botanists call such a flower self-pollinated.
The bean, then, is not often self-pollinated, but cross-pollina-
tion (meaning pollen from other flowers) usually occurs.
380
INTRODUCTION TO BIOLOGY
Ovules, — {Dor L) Carefully remove the corolla and thus expose
the pistil of a bean flower. Hold up to the light and notice a row
of opaque spots in the ovary. Then with a sharp knife or a razor
split the ovary lengthwise. A low-power microscope or a hand-
lens will make clear that the opaque spots are " seeds." In this
early stage, however, the term ovule should be applied to each one
of these structures which later grows into a seed. Inside each ovule
an embryo develops, and later when the seed sprouts the embryo
grows into a new plant.
Pollen-Grains. — (D or L) Examine some poUen-grains mounted
in a drop of water, using first low power and then high power of
microscope.
Fertilization, — When pollen-grains have been lodged on
the stigma of a flower they
soon swell, and each one
sends out a delicate tube,
which grows down through
the tissues of the style and
into an ovule in the ovary
(Fig. 103) . There the poU
len-tube meets a cell known
as the egg-cell. Then a
mass of protoplasm (really
a cell) passes from the end
of the pollen-tube into the
egg-cell. The union of
the two cells is fertili-
zation. Soon the fertilized
egg-cell divides into
many cells, and these
form an embryo within
seed.
Fig. 103. Diagram of a flower. C, calyx;
Co, corolla; a, anther on filament (/);
p, pollen-grains; at, stigma; pt, pollen-
tube; 8, style; 0, ovary; em, egg-cell
in ovule; E, fertilizing cell. (From
Besaey)
the ovule, which grows into a
Pea Flower, — (L) If material is available, make a brief examina-
tion of the flower of a pea, preferably of a sweet pea, in order to see
better some parts which are larger than in the bean flowers.
REPRODUCTION OF OROAIflSMS 381
Tfie Bean Pod or Fruit
337. Fruits. — After the stigma of the bean flower has
been pollinated by insects, the ovary of the pistil soon begins
to develop into the pod containing the seeds. Botanists
call the pod with its seeds & fruity and apply this name to the
structures containing seeds which develop from flowers.
Hence many things are known in biology as fruits which we
do not popularly call fruits ; for instance, a tomato, a bean
pod, a squash, or a cucumber is in botany just as much a fruit
as an apple, a peach, or an orange.
338. Study of a Bean Pod. — (L) On a full-grown bean plant
about six weeks old one may usually find pods (fruits) of various
sizes, ranging from one slightly larger than the pistil of the flower
up to the size of the fully developed pod. The stalk or stem
of the pod is the flower-stalk (pedicel), and its expanded end at
the point whwe it is attached to the pod is the receptacle of the
flower. In the younger pods, identify the calyx and the bracts of
the flower, which can still be seen at the stalk end; and at the
other end is the style. Between the caljrx and the style is the
ovary, which begins to elongate shortly after pollination. If pos-
sible, examine the ovary in a faded flower, just about the time
that the corolla is ready to fall off. Label sketches of young pods
so as to show what parts of the flower develop into the fruit.
Study a full-grown bean pod (use green pods, known as " string-
beans "). Sketch and label, naming the parts by comparing with
younger pods. The pointed end is the base of the style, most of
which, with the stigma, was pulled off by the falling corolla. Note
that the pod is composed of two similar valves fastened together
along the edges, which are called sutures (meaning seams). The
bean pod is bilaterally symmetrical. The style and the concave
curvature of the pod mark the edge or suture Where the seeds are
attached. Carefully split open a pod along the opposite edge and
note that some beans (seeds) are attached to each half. Is there
any regularity as to the number attached to either half? Note
that each bean is held in place by a short seed-stalk (funiculus).
Undeveloped ovules may be seen near the ends of the pod. The
382 INTRODUCTION TO BIOLOGY
part of the inner lining of the pod to which the seeds are attached
is known as the placenta.
Break a pod transversely in several places, and notice strong fibers.
On which edge of the pod are they ? Do you see any relation be-
tween the position of the strongest fibers and the natural curvature
of the pod? Why are the green pods called ** string-beans '' by
gardeners?
(D) Cut a fresh bean branch having young pods, and place cut
end in red ink. After a time note where the pods are colored and
draw your conclusions as to the functions of the fibers in the pods.
Is there any reason why these fibers should be more abundant on
one edge of the pod?
(D) Note markings on a bean where it was attached to the
seed-stalk. The scar left when the seed-stalk is pulled off is called
the hilum. At one side of the hilum (toward the stalk of the pod)
is a small translucent elevation with a slit-like marking. On the
opposite side of the hilum is a very minute pit known as the mi-
cropyle. When pollen-grains touch the stigma, as previously
described in § 336, the very small tube that grows from each pollen-
grain extends down the style and along the placenta of the ovary
to the micropyle of an ovule. Later, when fertilization is com-
pleted, each ovule develops into a sejed, and the entire ovary into
the fruit or bean pod. Make a diagram of a pod with seeds, and
show by a broken or colored line the path a pollen-tube must take
from the style to an ovule.
Tfie Bean Seed and its Germination
339. Varieties of Beans. — (L) Compare color, markings, and
size of specimens of some of the common varieties of beans grown
in gardens. (The school-museum should have a collection of the
most common varieties of beans arranged in small labeled bottles
or boxes.)
The variations in color, size, etc., of the seeds are no greater
than the variations of all parts of the plants which grow from
them : (1) Bean plants may be low (dwarf beans), or climb-
ing (e.g., lima beans) ; (2) they may have flowers of various
colors ; (3) the leaves may differ in shape, size, and color ;
REPRODUCTION OF ORGANISMS 383
(4) the pods may be rounded or flattened, short or very long
(in one variety two to three feet), green or yellow (so-called
'' wax beans "), with " strings '' (§ 77) or almost " stringless/'
and with many flavors; (5) some bean plants form edible
pods in six or seven weeks after planting (the early and extra-
early varieties), while others take a longer time (lima beans
are often killed by frost before the seeds are full size). These
are a few of the variations of beans which interest gardeners,
because all these are qualities sometimes desired. Look over
the descriptions of beans in a seed-catalogue, and note the
points emphasized in the descriptions of varieties offered for
sale.
340. Structure of a Bean. — (L) Use any large beans (limas,
" yellow six-weeks," " scarlet runner," and " golden-eyed wax "
are excellent) ; some dry, and some which have been soaked in
water over night.
Examine the surface markings. Locate the scar (hilum) and
the micropyle, which were described in the preceding lesson on the
pod. The translucent elevation seen in green beans near the hilum
and opposite the micropyle is colored in dry beans of many varieties
(dark brown in ** yellow six-weeks," light brown in *' golden-eyed
wax " beans). These conspicuous marks will aid in locating the
micropyle on the opposite side of the hilum.
Strip off the seed-coat from a water-soaked bean. The seed-coat
has two layers, which are easily seen in a green bean. The main
body of the bean consists of two thickened halves (seed-leaves or
cotyledons). These are stored with food for the early use of the
young plant that will grow from the seed.
Carefully separate the two cotyledons, and notice a pointed rod-
like body which is joined to the cotyledons. This is called hypocotyl
(Fig. 104, h). It is best seen in a bean which has begun to sprout,
for the hypocotyl then pushes through the seed-coat at the micro-
pyle. The part of the hypocotyl next the cotyledons will form the
beginning oif the sterriy while the pointed end will form the first root.
Before sprouting begins, it is difficult to see any line between the
stem and the root part of a hypocotyl ; and we simply call the entire
384
INTRODUCTION TO BIOLOGY
structure hypocotyl
until growth makes
it easy to distin-
guish between stem
and root.
The words eau-
liele and radicle
which are used in
some botanical
books are practic-
ally synonymous
with hypocotyl.
Joined to the
hypocotyl where
this is united with
the cotyledons is a
very short stem
with a pair of small
leaves. Between
these leaves is a
small bud. This
short stem with
leaves and bud con-
stitutes the epicotyl.
It will form the stem and leaves above the cotyledons. Some books
call the bud with the small leaves a plumule.
Fig. 104. Bean seed and seedlings,
e, epicotyl; h, hypocotyl; r, roots.
son.)
c, cotyledons;
{After Atkin-
341. The Bean Embryo. — Cotyledons, hypocotyl, and
epicotyl together constitute the embryo which may develop
into a bean plant. The part of the plant which develops
from each part of the embryo is as follows :
Bean embryo
consists of
Cotyledons — not very useful as foliage leaves o(
bean seedling, but stored with food.
Hypocotyl — forms stem below cotyledons and
root at its lower end.
Epicotyl — forms stem and leaves above coty.
ledons.
REPRODUCTION OF ORQANtSMS 386
As we shall see in later studies, the seed-coat is lost as the
embryo develops; it is therefore simply a structure for
protecting the embryo after the seed is out of the pod and
until germination is completed.
342. Germination or Awakening of Seeds. — A dry bean
seed shows none of the usual signs of being alive; but it.
soon revives when placed under proper conditions (with
moisture, heat, and oxygen from the air), and rapidly grows
into a young plant (called seedling, or plantlet). This
" awakening " or reviving and growth into a new plant is
conmionly called germination; and the seed is said to ger-
minate. The popular word " sprouting " usually means the
early stages of germination.
Germination of Beans. — (L) Beginning about two weeks before
this exercise, some beans should have been planted every other day
in soil (preferably in small boxes or flower-pots which can be taken
to the schoolroom). Plant about two inches deep, and keep the
soil moist and warm. Plant a few beans four, five, and six inches
deep, and some near the soil surface. Mark the position of these
with wooden stakes on which figures have been written with lead-
pencil. When some young plants (seedlings) are two inches above
the soil, and others just emerging, the materials are ready for the
following lesson.
Without pulling up any plants, carefully examine and compare
the various stages in order to determine where the parts of the
embryo seen in the seed are located in the seedling. What becomes
of the seed-coat ? The first joint or node of the stem is where the
cotyledons are attached ; the second at the leaves of the epicotyl.
Does the internode between these two nodes lengthen? Do the
cotyledons of the bean become leaf-like? Compare the size and
shape of the leaves of the epicotyl with those you have seen on a
large bean plant.
Make a series of sketches showing stages in the emergence of the
seedling from the ground. Leave space in note-book for adding
other and larger sketches of later stages as the plant develops for
several weeks.
2c
386 INTBOVUCTION TO BIOLOGY
In order to find out what happens between the time the seed is
planted and the seedling emerges from the soil, we must either dig
up seeds planted at various periods of time or we must study these
early stages of germination in seeds which are germinated without
contact with soil. This latter is best, because the particles of soil
cling to the seeds and make them more difficult to study.
To germinate seeds without soil, place them between layers of
cotton batting, sphagnum moss, sawdust (some kinds are too acid),
blotting-paper, filter-paper, or other soft papers. Keep moist, not
wet, and warm. Or simply place seeds on a few layers of soft
paper on a flat dish (such as a dinner-plate) and keep moist, warm,
and covered. Sand or cotton under the paper will help hold
moisture.
(L) Examine various stages of bean seedlings grown without
soil. Compare carefully with seedlings growing with their roots
in soil, and make sketches of the chief stages. See Fig. 103.
Conditions of Germination, — It is easy to demonstrate
that water, oxygen (from air), and proper temperature are
necessary for germination.
Water, — That water is necessary requires no special ex-
perimental proof, for every one knows that seeds kept dry
remain dormant, and that seedlings do not " come up "
in gardens when the soil is dry, as in midsummer. Many
hard-coated seeds have special ways of letting in the necessary
water, such as the spongy mass at end of castor-bean, and
the holes in a coconut. Other seeds must remain in the soil
for months until freezing or decay of seed-coats causes cracks
into which water can enter. Gardeners often crack certain
hard seeds, like peach and various nuts, and they file or cut
notches in others. Certain seeds should be soaked and
softened in hot water before planting.
Proper Temperature, — Any one who has ever been in-
terested in a garden knows that warm weather is necessary
for the germination of most seeds, but that some kinds of
seeds will germinate in early spring when the soil is still cold.
REPRODUCTION OF ORGANISMS^ 38T
By simply placing some seeds on a moist paper in an ice-box
and others of the same kind in warmer places one can show
the effect of temperature on germination, and after many
trials it will be found that there is a best or optimum tem-
perature for each kind of seed. Seed-catalogues and books
on gardening usually give the necessary information regard-
ing best temperatures for germinating common vegetable
and " flower " seeds.
Air J Oxygen. — That the oxygen of the air is necessary
might be expected because a germinating seed is a living
plant, and we have learned that all protoplasm must have
oxygen. It can be proved by placing seeds in a bottle and
pumping out the air with an air-pump, or by filling the bottle
with pure nitrogen gas made with a chemist's generator.
That carbon dioxide is formed in germinating seeds can be
proved by the lime-water test — simply stand a small cup
or vial containing lime-water in a larger, carefully stoppered
bottle containing germinating seeds. Or use the method
described in § 25, suspending seeds in a bag of netting during
germination, and then after lifting out the bag, pour lime-
water into bottom of the jar.
343. Other Flowers, Fruits, and Seeds. — If time permits, it
will be worth while to compare the bean reproductive structures
(flowers, fruits, and seeds) with those of other common plants,
such as pea, squash, corn, castor bean. Directions for the study
of these are given in the authors' " Applied Biology,** §§ 135-145,
189-208, 212-217.
Reproduction of Ferns
344. Structure of a Fern Plant. — The stems of the com-
mon ferns are either prostrate on the soil surface or under-
ground, but the tropical tree-ferns, often kept in greenhouses,
have upright stems. From the horizontal stem of a com-
388
INTRODUCTION TO BIOLOGY
.^s^?^^?^-
mon fern the leaves (fronds) grow up each summer, and there
are small root-like structures extending from the stem into the
surrounding soil. Examine a fern
plant and compare its parts with
thoge of a bean or other flowering
plant.
346. Fern Spores. — Many fern
leaves have on their lower sur-
faces certain elevations, ridges, or
folds quite unlike anything found
on the leaves of flowering plants.
These peculiar structures contain
spore-cases (sporangia), and these
form spores.
Spores scattered by the bursting
of mature spore-cases absorb water
from moist soil, germinate, and
grow into a heart-shaped and leaf-
like plant, called a prothaUium
(Fig. 105). It may be more than
one-eighth inch across its widest
part.
346. Sex-Organs of Ferns. —
On the under side of a fuUgrown
prothaUium there are ovaries and
spermaries.* Each ovary pro-
duces an egg-cell or ovum, and
each spermary forms many spiral
sperm-cells with cilia (Fig. 105).
The lashing movements of the cilia cause the sperm-cells to
swim in drops of water to the opening of the tube-like ovary
* Spermaries of ferns are often called antheridia, and ovaries archegonia.
These names are given here merely for reference to books of botany.
Fig. 105. A, fern prothaUium
developed from a spore which
was formed on a fern leaf,
o, ovaries; «, spermaries;
r, root-hairs. B, a sperm-cell
from spermary of fern. C, a
young fern plant growing
from a fertilized egg-cell in
an ovary on a prothaUium.
{After Parker.)
REPRODUCTION OF ORGANISMS 389
and thence in to meet the egg-cell. One sperm-cell pene-
trates an egg-cell, and this is fertilization.
The fertilized egg-cell undergoes repeated division and
forms an embryo which soon grows into a young fern plant
(Fig. 105). The prothallium withers, the fern plant grows to
maturity, produces spores, which germinate and grow into
new prothallia. Thus the ordinary fern plant reproduces
asexually by dividing off spores, which form the plants known
as prothallia, and these reproduce sexually (i.e., by means of
egg-cells and sperm-cells) and form new fern plants. The
complete life-history includes two generations, the fern plant
and the prothallium ; and so ferns are said to have aUemation
of generations. So far as superficial observation goes, the
ordinary flowering plants appear to have only one generation,
namely, the plants with flowers producing seeds which grow
into new plants able to produce seeds. But in advanced
courses of botany the student learns that inside the pistil of
flowers are microscopic groups of cells which represent the
prothallium that lives independently as a stage in the life-
history of ferns and other lower plants.
EMBRYOLOGY OF THE FROG
The reproduction of the bean plant has been described as
illustrating that of the highest plants (seed-plants), and
now we turn to study briefly the development of some of the
highest animals (vertebrates). While the fishes are the
lowest of this group, it will be more convenient to center our
study of vertebrate life-history around the frog, with
whose structure and physiology we have become familiar
through Chapter VI.
In considering the work of the organs of the frog, we gave
attention to those which are necessary for the life of the in-
390
INTRODUCTION TO BIOLOGY
dividual frog, that is, all organs except the reproductive
organs which are necessary only for the continuance of the
frog species. The work of the reproductive organs of the
frog (as of all other organisms) is the production of new in-
dividuals ; and to the general facts in this Une we shall now
give some attention.
347. Egg-Cells and Sperm-Cells. — In the study of the
structure of the frog (§ 89), we noted the position and form of
the essential reproductive organs (ovaries and
f I «\ spermaries) and their ducts by which egg-cells
f 1 \ [ (from ovaries) and sperm-cells (from sperma-
* ' ' • ries) are conducted out of the body into the
water in which the animals live. From the
ovaries and spermaries of frogs the egg-cells
and sperm-cells are discharged in early spring.
The eggs develop in the water and hatch as
tadpoles. Then, after a few weeks or many,
depending upon the species, they develop legs,
lose their tails by a process of absorption, and
become small frogs. This change from tad-
poles into frogs is called metamorphosis.
The ovaries in a young frog are masses of tis-
sue with numerous small egg-cells. Each egg-
cell is a spherical mass of protoplasm with a nucleus near
its center. ' As the eggs grow larger, each one accumulates
granules of a material known as yolk; and after a time
the yolk comes to occupy one hemisphere of the egg,
while the protoplasm is concentrated in the other. Frogs'
eggs examined soon after they are laid in water are seen
to be black (with pigment) in one hemisphere, and whitish
(due to yolk) in the other. The black hemisphere con-
tains most of the protoplasm. Each egg is surrounded by
an envelope of transparent jelly, which was secreted by the
Fig. 108. Va-
rious forms of
sperm-cells.
1, 2, 3, mam-
malian; 4, am-
phibian; 5,
crustacean.
REPRODUCTION OF ORGANISMS 391
cells of the oviduct as the egg passed from the ovaries to
the exterior.
The sperm-cells have the form shown in Fig. 106. The
thickened part is chiefly nucleus. The tails of living sperm-
cells vibrate rapidly and enable the cells to swim in the water,
into which they are discharged.
348. Fertilization. — Before an egg-cell can develop to
form a tadpole it must be entered by a sperm-cell. Soon
after the egg-cells of a frog are discharged into the water,
they are approached by the swimming sperm-cells and each
egg is entered by a sperm-cell. The sperm-cell then becomes
Fig. 107. Diagrams illustrating fertilization of an egg-cell by a sperm-cell.
At e, nucleus of matured egg-cell; «, a sperm-cell ready to enter. B^
sperm-cell entered and transformed into sperm-nucleus (s). C, sperm-
nucleus and egg-nucleus united, fertilization complete. Z>, division
leading to two-cell stage. The nuclei are represented as having only
two chromosomes; but those of most sperm- and egg-cells have more.
a second nucleus, called sperm-nucleus. This nucleus ap-
proaches that of the egg-cell, and the two nuclei unite. When
this has occurred, the egg-cell is said to be fertilized^ and the
entering of a sperm-cell and its union with the nucleus of the
egg-cell is cdXled fertilizaiion (see Fig. 107).
349. Division of the Egg-Cell. — As soon as the egg-cell
is fertilized, it prepares for division into two cells.* In
warm spring weather the first division is usually completed
within two hours after the egg-cells are laid in the water, and
* For details concerning division, see "Applied Biology," § 59.
392
INTRODUCTION TO BIOLOGY
the egg is then in the two-cell stage (Fig. 108, F), Each cell
appears exactly like the original egg-cell, but is one-half the
size. In another hour
each cell of the two-
cell stage will have
gone through the same
processes of division,
and the four-cell stage
will be reached. Such
divisions of cells occur
about every hour ; and
so, in succession, stages
of two, four, eight, six-
teen, and thirty-two
cells are formed (Fig.
109). Later, some cells
may get slower in di-
vision, and hence the
stages only approxi-
mate 64, 128, 256, and
more cells, multiplying
by two when all the
cells have divided.
Usually within two or
three days the egg has
reached a stage consisting of a spherical mass of many
hundred cells ; and it is then ready to arrange the cells so as
to form the body of a tadpole.
360. Later Development. — The changes in external form
of developing frogs' eggs should be observed, either in eggs
preserved in formalin-solution, or in living eggs (only in
early spring) kept in aquaria (e.gf., fruit-jars) and examined
daily during development. In brief, the changes are as
Fio. 108. Diagrams illustrating stages in
division of cell (A) into two (F)- w.
nucleus; c, centrosome; cw, cell; ch, chro-
mosomes. (From WiUon.)
REPRODUCTION OF ORGANISMS
393
follows : The spherical mass of cells formed by the nmnerous
divisions of the egg-cell becomes elongated, the various
organs are formed by complicated changes which cannot be
described in the limited space of this book, and gradually
the embryo becomes a tadpole. Finally, the tadpole hatches,
that is, it breaks through the jelly that has surrounded the
egg throughout the development.
The tadpole at hatching appears larger than a fertilized
frog egg; but if dried would not weigh more, for no food
Fig. 109. Stages in division or cleavage of frog's eggs. The figures indicate
the number of cells. (From Thomson, after Ecker.)
can be taken until after hatching. The larger size of the
tadpole, as compared with the egg, is due to water absorbed
during development.
The time necessary for a fertilized egg-cell to develop into
a free-swinmiing tadpole varies with the temperature of the
water. Also, the eggs of some species develop faster than
others; toads' eggs will hatch within three or four days
after being laid and those of wood-frogs develop almost as
rapidly.
361. Metamorphosis. — The time necessary for a tadpole
to become full-grown (Fig. 110) and to metamorphose into
394
INTRODUCTION TO BIOLOGY
a toad or a frog varies with species, and with food and tem-
perature of water. Toad and wood-frog tadpoles may meta^
morphose within one or two months after
hatching. Such rapid development is impor-
tant, for these animals often lay their eggs in
temporary pools which become dry in late
spring or early simimer. Bull-frogs and some
green frogs may live in the tadpole stage at
least a year, for in early spring there are in
permanent ponds many large tadpoles show-
ing no signs of developing legs and shortening
tails and these are at least one year old. Prob-
ably some of these will become frogs in the
second summer when somewhat over one year
old, while others may live still another year in
the tadpole stage.
The nature of the metamorphosis of a tad-
pole into a frog is popularly misunderstood.
The tadpoles are said to " lose their tails," and
this is often taken to mean that the tails
drop off. This is not true. In collecting
foreleg; p, exit tadpoles in ponds one often finds specimens
l^Tz. left iiT^! with short stubby tails and well-developed
{FromAua der legs, and others showing various intermediate
stages between these and tadpoles with long
tails and no legs. The explanation of these different condi-
tions is that after the legs develop the tail is absorbed. First,
the tissues inside the end of the tail are disintegrated by
the white blood-cells, which '' eat " the particles of tissue
and carry them back into the body. In a short time after
this process starts, the tip of the tail appears withered.
The white blood-cells continue this process of disintegration
and removal of tissues until the tail has entirely disappeared.
Fig. 110. Full-
grown tadpole
dissected on
leftside. Intes-
tine coiled. /,
REPRODUCTION OF ORGANISMS
395
Thus instead of. the tail dropping ofif, as is commonly be-
lieved, its substance is gradually carried back into the body
by internal processes of
absorption, and the
materials are used in
building other tissues.
352. Embryonic and
Larval Development. —
The development of
any fertilized egg to the
hatching stage is called
embryonic development;
and the scientific study
of such stages of the
frog and other animals
is animxil embryology.
The tadpole is often
called a larva, and its
changes constitute lar-
val development.
All species of back-
FiG. 111. Transformation of tree-frog tad-
pole in 24 hours. Natural size. {From
Cornell Leaflets.)
boned animals have embryonic development; but a large
number do not have a larva, for their eggs develop directly
into miniature animals resembling the adults. For example,
it is well known that birds are hatched as small birds, and
common mammals at birth show the characteristics of their
species. All young animals which at hatching are quite un-
like their parents (e.g., as tadpoles are unlike frogs, and cater-
pillars are unlike butterflies) are said to have a larval develop-
ment.
363. Oviparous and Viviparous Development. — All such
animals as frogs, birds, turtles, fishes, etc., which lay eggs
that develop outside the body of the animal that produced
396 INTRODUCTION TO BIOLOGY
them, are called oviparoiLs. Such eggs are usually protected
by special coverings or shells. We shall see later that such
oviparous development is very common in the animal king-
dom.
In many species of animals, eggs are retained in the oviducts
or other specialized cavities until embryonic development
is completed, and the young animals are " bom alive " ; by
which we mean that as organisms ready for an independent
existence they are expelled from the cavity in which embry-
onic development occurred. Such internal development is
viviparous. It occurs in all mammals, except the duck-bills
of Australia; in some snakes; in some salamanders; in
some fishes ; in some* insects ; and in other kinds of lower
animals.
There is a great advantage in viviparous over oviparous
development in the protection afiforded the eggs and em-
bryos. Hence relatively few eggs need be produced. It is
well known that many fishes and other oviparous animals
produce an astounding nimiber of eggs, and that vast num-
bers of the eggs and young are destroyed. Sharks illustrate
the advantage of viviparous development for fishes. Few
eggs are formed, and these are retained in the oviducts,
not only until the young animals are fully formed^ but for
many months, until they have grown to be several inches long.
They are then born (expelled by muscular contraction of these
oviducts), and are well able to shift for themselves. Thus a
few sharks' eggs well protected during development will
perpetuate the species as successfully as would hundreds of
eggs forced to develop oviparously and exposed to attacks of
numerous enemies.
A similar case is found among salamanders, which are near
relatives of the frogs and toads. Our common salamanders
lay each spring large masses of eggs. In a species which
REPRODUCTION OF ORGANISMS
397
lives in the mountains in Europe, each female forms but two
eggs in a year, and these are retained in the oviducts until
developed into young
salamanders ready to
care for themselves.
364. Reproduction
of Fishes. — In all
species of vertebrates
there are both male and
female individuals; and
new individuals always
originate from egg-cells
fertilized by sperm-
cells.
The eggs of most
fishes are laid
('^ spawned ") in shal-
low water in quiet
places, and sperm-cells
discharged into the
water fertilize the eggs.
The enormous loss of
eggs and young fishes
under natural condi-
tions has led to arti-
ficial fish-culture under
the control of various
states and the United
States Bureau of Fish-
eries. The methods of
errib
Fig. 112. Stages in development of a fish.
hU germ-disk, remainder of the egg is yolk;
6w6, developing embryo; i/«, yolk-sac.
{From Parker and HaaweU.)
hatching are a scientific application of nature's way. Fishes
are caught by nets at the breeding season, and pressure on their
ventral surfaces causes extrusion of eggs a^d sperm-cells . These
398 INTRODUCTION TO BIOLOGY
are mixed in water and allowed to stand in shallow pans or
water-tight boxes until the sperm-cells have by swimming
reached and penetrated the egg-cells (one sperm for each
egg), and fertilized them. Later, the fertilized eggs are placed
in boxes arranged so that water runs over them while they
develop. After hatching, the young fish (called fry) are
kept in small pools or tanks,where they are easily fed and pro-
tected until large enough to care for themselves in rivers and
ponds where their enemies live. Of course many are killed
after they are turned loose, but artificial hatching is advan-
tageous in that the developing eggs are protected.
The spawning habits of some fishes are most remarkable.
Some salmon ascend the Columbia River from the sea for
more than a thousand miles, at an average rate of 3 to 4 miles
a day. After depositing eggs and sperm-cells, they all die ;
and no old fish lives to lead the young ones downstream to
the sea. This single spawning occurs after the salmon are
at least three years old. It is not true, as sometimes stated,
that they always go back to the river where they were hatched ;
but they probably do not go far from the mouth of the river,
and hence are likely to ascend the same river when fully
developed and ready to spawn.
The shad of our Atlantic Coast is another example of a
fish that ascends rivers- to spawn ; but this fish lives to return
to the sea after spawning. More than 200 million shad
eggs are artificially hatched each year.
The common river eels migrate downstream in the autumn
to spawn in the sea, and after spawning in deep water, the
old eels die. Hence adult eels never migrate upstream. In
spring, vast numbers of young eels, about one year old, ap-
pear below dams and waterfalls. A female 32 inches long
may have more than ten million eggs. The life-history of
eels wacs a complete riddle until about twenty years ago, when
it was found that the eggs are laid in deep sea-water.
REPRODUCTION OF ORGANISMS 399
Codfish spawn near the shores of New England between
December and April. The hatcheries liberate annually more
than 75,000,000 young fry. It is easy to collect the cod eggs
for hatching, for in a 20-pound female the two ovaries (popu-
larly called " roes '') contain more than 2,500,000 eggs,
which are so small that a quart bottle will hold about 335,000.
Think of how abundant codfishes would be if all the eggs of
a thousand females were to hatch and grow to maturity,
and one-half of these were to be equally prolific females.
However, since cods do not appear to be either increasing or
decreasing rapidly, we are justified in concluding that, on the
average, two eggs from each female produce mature individ-
uals. The others are destroyed by enemies or die from dis-
eases. This is a good illustration of the intensity of the
struggle for existence, which, to a great extent, affects all ani-
mals and plants.
355. Reproduction of Higher Vertebrates. — The early
stages of the embryonic development of reptiles, birds, and
mammals have a general similarity to those of the frog (§59).
In all cases, the fertilized egg-cell divides into numerous cells
which then form the body of the embryo.
It is a significant fact that there is great similarity in the
early stages of all vertebrates. This is illustrated by Fig.
113, in which in parallel columns are early, intermediate, and
late embryos of a fish, a salamander, a reptile, a bird, and a
mammal. In the early stages there is so great similarity
that only specialists in zoology could distinguish between
these embryos ; but as development proceeds there is more
and more differentiation, and the final stages at birth and
hatching are easily identified as fish, bird, etc.*
* To Teachers : If class time allows, describe the gill-slits as the most
remarkable of similar structures in vertebrate embryos. See "Applied
Biology," § 364.
400
INTRODUCTION TO BIOLOGY
Fig. 113. Similarity of early stages of vertebrate embryos (in column I) and
differentiation before birth or hatching (colunm III). {From Romanes^
after Haeckel.)
BEPRODUCTION OF ORGANISMS
401
366. Bird Development. — All species of birds are ovipa-
rous (external development) ; and the eggs require incubation.
For this a certain temperature is essential (about 40° C. or
104** F. for hen's eggs) . The eggs are fertilized soon after they
leave the ovary and enter the oviduct, and cell-division goes
on for about a day while the eggs are passing through the
duct to the exterior. But
soon after an egg is " laid "
it becomes cooled to below
the normal temperature
and development stops.
Within a variable number
of days, the development
may start again, if the egg
be warmed to the proper
temperature. In natural
conditions this is provided
for by the instinct which
causes female birds (some-
times the male) to sit on or brood the eggs. The feathers pre-
vent rapid loss of the heat afforded by the warm ventral surface
of the body of a brooding bird. This brooding instinct usually
appears soon after a female bird has laid the eggs which in a
given season have developed in the ovary. In many wild birds
such a season of egg-laying comes only once a year ; in some
species two or three broods of eggs may be laid in a summer ;
and the well-fed domesticated hen may lay from 100 to more
than 200 eggs per year, if not allowed to waste time and
energy by brooding after each set of 10 to 20 eggs, as they
instinctively do. The eggs of birds are large because they
have a great store of food (yellow " yolk," and the " white "
or albumen) for nourishment of the embryo during the de-
velopment. The eggs in the ovaries of young birds are small
2i>
Fig. 114. Diagram of bird's egg. yk,
yolk; abl, white or albumen; hi, germ-
disk; ch, thickened albumen, which
holds yolk in position; sh.m, two
shell membranes. (From Parker and
Haswell.)
402
INTRODUCTION TO BIOLOGY
spherical cells, but as they mature the storage of food causes
enlargement. For example, in an ordinary hen's egg the
" yolk " with its inclosing yolk-membrane is about one
inch in diameter, but in a young ovary it is a microscopic
cell. The *' white '' or albumen which surrounds the yolk
and also the shell are secreted around the egg as it passes
through the oviduct on the way to the exterior. Obviously,
the *' yolk " is the real egg, corresponding to a frog's egg, and
the " white " and shell are later additions formed like the
jelly around frog's eggs.
Careful observation of the " yolk " of a bird's egg will dis-
close a white spot on its upper surface. To see this, place a
fresh egg on some cotton or sawdust, care-
fully chip the shell with forceps, or cut
with scissors, making a hole about one
inch in diameter. The white spot on the
'' yolk " is the germ-disk from which the
embryo will develop; and all the other
material inside the yolk-membrane is food.
Hen's eggs kept under sitting-hens, or in
incubators, for 15; 24, 36, 48, and 72
hours, and then opened (as directed above)
will show a series of stages in the growth
of the embryo in the germ-disk.
Eggs which have been incubated for
several days show the embryo surrounded
by membranes in part of which there are
blood-capillaries). The blood in these
is pumped by the embryo's heart, which
begins to beat on the second day of in-
cubation. The purpose of circulating
blood in the blood-vessels of membranes outside the embryo's
body is (1) obtaining food from the yolk and later from the
Fig. 115. A chick
embryo of one day's
incubation. 6, be-
ginning of brain; h,
head; v, vein to yolk;
n, neural grove (later
spinal cord); p, pro-
tovertebrse; s, primi-
tive streak. {From
Marshall.)
REPRODUCTION OF ORGANISMS 40S
" white," (2) obtaining oxygen from air which filters in through
the pores of the shell, and (3) the discharge of carbon dioxide
made by the developing embryo. The importance of the
two later processes, which considered together constitute
the respiration of the egg, is shown by the fact that the em-
bryo dies from asphyidation (lack of oxygen and excess of
carbon dioxide) if the pores of the shell become filled, as when
coated with albumen from another egg accidentally broken.
Hence, poultry keepers must take great care to keep eggs
intended for hatching clean before and during incubation.
The time for incubation varies. Hens' eggs hatch in three
weeks, ducks' eggs in four weeks, and other birds have either
shorter or longer incubation. No one knows why the egg of
one species of bird requires more time than does that of
another.
357. Reptile Development. — Some snakes, the alli-
gators, and all turtles are oviparous, and their embryos
are like those of birds. Incubation is due to the sun's
heat and not to brooding by parents. Some snakes and
lizards are viviparous, the eggs being retained in the ovi-
ducts until fully developed into young animals which are
then ejected by muscular contraction of the oviducts. The
eggs of these viviparous reptiles are similar to the oviparous
ones, and contain much stored food (yplk) to be used by the
developing embryos. Among viviparous snakes the garter-
snake sometimes gives birth to as many as forty-five in one
brood ; the rattlesnakes produce 9 to 14 ; the copperhead, 7
to 9 ; and the water-moccasin, 7 to 14.
358. Mammalian Development. — We have already (in
§ 353) noted the advantages of internal or viviparous develop-
ment over the external or oviparous method ; and that while
cases of internal development occur among animals of many
groups, it is in the mammals that we find vivipary universal.
404 INTRODUCTION TO BIOLOGY
with the one exception of the Australian duck-bill or duck-
mole, which lays eggs.
In adaptation to viviparous development part of the ovi-
ducts of the mammals are expanded and fitted for holding
eggs during their embryonic development. Such an ex-
panded part of an oviduct adapted for developing embryos
is called a uteriLS or womb. In some low mammals, like
kangaroos and opossums, there is a right and a left uterus
formed by expansion of part of each oviduct. These lie in
the same position as the oviducts of frogs. In higher mam-
mals the right and left uteri grow together during embryonic
life, and so there is a single uterus with a tube (Fallopian) ex-
tending to each ovary (right and left). Egg-cells formed and
discharged by either right or left ovary pass through the corre-
sponding tube into the uterus and there develop into embryos.
In all mammals the egg-cells discharged are fertilized near
the ovaries in the Fallopian tubes by sperm-cells which have
arrived there by swinuning through
the secretions on the living mem-
branes of the uterus and tubes.
The fertilized egg begins to divide
at once and may have undergone
Fig. 116. Rabbit egg in two- considerable development when,
nScieur^^"^*^^ '*''^''" "*' ^f^^^ ^ f^w d^ys, it slips from the
tube into the uterus.
The number of egg-cells fertilized at one time varies in
different species. It is well known that many domesticated
animals (e.gf., sheep, cow, horse) usually have one offspring
at a time; but some occasionally produce two (twins) or
even three (triplets). Others commonly produce many
young at a time (e.gf., dog, cat, pig, rabbit, mice). The num-
ber of young produced indicates the number of egg-cells
which were matured and fertilized.
REPRODUCTION OF ORGANISMS
405
The period of development in the uterus from fertilization
to birth of the young is commonly known as gestation or
pregnancy; and the length of time is highly variable. It is
approximately 21 days in guinea-pig, 30 days in rabbit and
squirrel, 55 days in cat, 62 days in dog, 3 months in lion,
4 months in pig, 5 months in sheep aitd goat, 6 months in
bear, 9 months in cow, over 9 months (280 days) in human
species, 10 months in whale, 11 months in horse, 14 months
in giraffe, and 22 months in elephant. These are simply
illustrations selected from familiar mammals.
In order to provide for the nutrition and respiration of
embryo mammals, a complicated connection is made be-
tween the blood-system of
the embryo and that of the
mother. Figure 117 shows
a rabbit embryo with its
surrounding membranes.
These are abundantly sup-
pUed with blood-vessels
connected with the em-
bryo's heart. "Figure 118
shows the position of
an embryo mammal in a
uterus. The darkly shaded
area around the embryo re-
presents lining tissue (epi-
thelium) of the uterus, arid
this tissue receives its blood-
supply from the heart and
arteries of the mother. The
tree-like processes shown in
Fig. 118 are further outgrowths of the irregular processes
shown in black on the outer membranes of the embryo rep-
FiQ. 117. Rabbit embryo of twelve
days' development, with its surround-
ing membranes which serve for attach-
ment to the lining of the uterus. 1,
nose; 2, eye; 3, ear, 4, fore-leg; 5, hind-
leg; 6, backbone; 7, umbilical cord;
8, amnion (protective membrane);
9, allantois (absorber of food and oxy-
gen) ; 10, tree-like processes that unite
with the uterus; 11, sac filled with
fluid (From Marshall.)
406
INTRODUCTION TO BIOLOGY
resented in Fig. 117.* These tree-like processes receive their
blood-supply from the embryo's heart through blood-vessels
in the umbilical cord at-
tached to the embryo at the
umbilicus or navel. As a
result of this close attach-
ment of the membranes of
the embryo and the lining
of the uterus, the blood-ves-
sels of the two are near
enough to allow osmosis.
From the maternal blood-
capillaries foods and oxygen
osmose into those of the em-
bryo, and excretions of the
embryo's cells pass into the
maternal blood. Solid bodies,
like red blood-cells, cannot
pass from the maternal to
the embryo's blood ; and the
blood-cells in the embryonic
blood-vessels are formed
from certain cells belonging
to the embryo. However,
the important point is that
food, oxygen, and excretions
osmose between the maternal
and embryonic blood-vessels in the membranes which at-
tach the embryo to the wall of the uterus.
The membranes which attach the embryo to the uterus
constitute the placenta. It normally separates from the
uterus after the birth of an embryo, and is then itself dis-
charged by muscular contractions of the uterus.
Fig. 118. Diagram of a mammalian
uterus, showing attachment of an em-
bryo to the lining (black in the fig-
ure). The lunbilical cord, from the
ventral surface of the embryo's abdo-
men, extends to the tree-like proc-
esses imbedded in the lining of the
uterus. The black lines in the um-
bilical cord indicate arteries and veins
connected with the embryo's heart.
The openings of the uterus shown at
upper right and left are the Fallopian
tubes leading to the ovaries, while
the lower opening is the mouth of
the uterus through which the mature
embryo is finally expelled by muscu-
lar contraction. {From Marshall.^
REPRODUCTION OF ORGANISMS 407
The fact that an embryo attaches to the wall of the uterus
as above described and is thus enabled to get food from the
blood circulating in the tissues of the mother explains the large
size of many mammals at birth. The eggs of all mammals
are transparent cells of microscopic size at the beginning of
development.
It should be noted that the entire time of gestation is not
occupied with the formation of the embryo's organs. For
example, an embryo may develop in two or three months so
that it has the form and structure of an adult, but may be
held in the uterus many more months in order to afford pro-
tection and nourishment while it grows larger and stronger.
This later stage after the organs are formed is often called a
fcstus, so as to reserve the word embryo for the early stages
when the organs are developing.
In the limited time available in this course we cannot do more
than study the mere outlines of mammalian development
as stated in the foregoing. There are many facts in this
line which are especially interesting because of the light which
they throw on human life, and for these the reader must be
referred to special books, and to college courses in embryology
of animals.
All the general statements made above concerning mam-
malian development apply to human development. The
great similarityof the embryos is shown in Fig. 113.
HEREDITY AND EUGENICS
359. Offspring like Parents. — An account of the reproduc-
tion of organisms would be incomplete without some reference
to heredity. It is a well-known fact that offspring tend to
resemble their parents closely. We commonly speak of
characteristics which previously appeared in the parents or
even earlier progenitors as inherited or due to heredity.
408 INTRODUCTION TO BIOLOGY
What may be Inherited, — It is a popular saying that
" like tends to produce like." This means that each animal
and plant has the power of transmitting its own general
characteristics to its offspring, e.g., a frog transmits frog
characteristics, a bird those of its own species, and so on.
Such characteristics which are part of the constitution of
organisms are often called germinal^ which means in the ova
and sperm-cells.
Characteristics developed during the lifetime of any indi-
vidual, dating from the fertilized egg, are said to be acquired.
Any change in structure due to use or accident, e.g., develop-
ment of muscles by exercise, or loss of organs by accidents or
surgical operations, produces an acquired characteristic.
Stating briefly the essential facts of heredity as now known,
characteristics acquired during the lifetime of individuals are
not transmitted in heredity, while germinal ones are capable
of inheritance. For example, the horns have been removed
from many cattle, the appendix from many human individuals,
tails from sheep and certain breeds of dogs ; and in no case
has the removal of any organ from a parent caused the birth
of offspring without the organ. In short, such acquired
characteristics are not inherited. On the other hand, a cow
which was germinally hornless (i.e., born without ability to
grow horns) would probably transmit the tendency towards
absence of horns to a considerable percentage of her off-
spring, and these, in turn, would tend to produce hornless
offspring. In fact, the breeds of hornless cattle, which are
now becoming popular among farmers, have been developed
by selecting for breeding certain individuals born without
the beginnings of horns. Likewise, dogs and cats bom with
short tails are likely to transmit that germinal characteristic,
which in some unknown way is carried in the reproductive
cells from parent to offspring.
REPRODUCTION OF ORGANISMS 409
The above paragraph states the facts verified by careful
observation in hundreds of cases. In fact, the numerous
varieties of domesticated animals and plants have originated
by man's selection of individuals which during their em-
bryonic history began to develop peculiarities. If these
peculiarities make the individual decidedly different from
its parents, a new breed might be originated, as from the
first hornless cow from horned ancestors. Usually, however,
the peculiarities are little things which, if selected by man,
will make an improvement in the breed. Hence the scientific
farmer is continually watching for young animals which show
some slight improvement over their parents; and he goes
through his fields in search of corn and other plants which
are better than the others. This, in brief, is the secret of the
remarkable improvement in almost all kinds of farm animals
and cultivated plants in the past fifty or one hundred years.
360. Inheritance from Both Parents. — It is obvious that
in cases of asexual reproduction of organisms (§§ 331-333)
and of the development of certain insects and plants from
unfertilized eggs, there is but one parent from which to in-
herit. This is one explanation of why plants propagated
from cuttings and grafts remain true to their variety, that
is, the young plant has the characteristics of its one parent.
All animals and plants developed from fertilized eggs in-
herit equally from both parents. Here are a few well-known
examples : If the pistils of flowers of a tall pea plant be arti-
ficially pollinated with pollen from a flower on a dwarf variety
of peas, the resulting hybrid seeds will produce tall plants.
This appears to be inheritance from the tall parent only, but
the dwarf characteristic is merely hidden. The hybrid tall
plants, pollinated from their own flowers, will produce seeds
of which 25 per cent will form dwarf plants, 25 per cent pure
tall plantS; and 50 per cent that in later generations will give
410 INTRODUCTION TO BIOLOGY
equal proportions of dwarf and tall. This proves that there
was equal inheritance from both the dwarf and tall parents
used in making the hybrid peas.
If black female cavies (" guinea-pigs ") be mated with
white males their hybrid offspring will be dark colored as
shown in Fig. 119, C, Now, if these hybrid offspring are
paired, their offspring will average 25 per cent pure white,
25 pure black, and 50 that are black, but really mixed be-
cause they are able to produce equal numbers of white and
black. Thus it appears that the hybrids shown in Fig. 119, C
inherited colors from both parents shown in A and B, but
the white is concealed in the first generation. Specialists
on heredity call the obvious black dominant and the hidden
white is recessive.
Another interesting proof that there is inheritance from
both parents is the fact that a long-haired white male cavy
and a short-haired black female will produce long-haired
black offspring, the color from the mother and the long hair
from the father.
Facts such as those stated above for peas and cavies are
now known in many cases where it is possible to get hybrids
between parents which are markedly unlike, such as the
difference of color of the cavies or of height of the peas.
The peculiarities of one parent are often so dominant that
the hybrids appear to inherit from only that parent ; but
later generations, or even other structures like the long
hairs of the white cavy, show that the other parent was
equally represented.
Such cases as those of peas and cavies, in which the char-
acteristics of each of the two parents that form hybrids
become segregated in 25 per cent of the offspring of each
succeeding generation are examples of Mendelism. This
principle of heredity was discovered by Mendel, an Austrian
Fig. 119. A, white male cavy; B, black female and three black off-
spring by black male; C, two hybrid offspring of the white male and
black female shown in B, and black dominant; Z), offspring of such
hybrids as in C, one in four are white and three black. Two of the
three black will produce one white to three black offspring, but one is
pure black. {From Castle's ''Heredity,'* Appletons.)
412 INTRODUCTION TO BIOLOGY
monk, who for many years cultivated and hybridized peas
in his garden and published his results in 1865.
Since an offspring inherits from each parent, it follows that
the egg-cell and the sperm-cell which unite to produce the
new individual are the bearers of heredity. But how these
simple cells can determine that the new individual into which
they develop shall resemble its parents is entirely a mystery
to biologists.
361. Eugenics. — There are many other principles of
heredity which concern inheritance from parents. Many
of them are applicable to human life, and within recent
years there has been great popular interest in eugenics,
which means the application of the known laws of heredity
in order to produce more perfect human individuals. At
present most of the writers on eugenics are urging that ab-
normal humans, such as the deformed, the weak-minded,
the insane, and the physically weak, should not become par-
ents because the chances are that a considerable number
(but not all) of their descendants within a few generations
will inherit the unfortunate characteristics. All this ap-
plies only to characteristics that are inborn or congenital,
for, as explained in § 359, changes which occur after birth could
not be transmitted to offspring. Of course, insanity and
idiocy may not show at birth, but there may be an inherited
weakness of nervous organs which sometimes leads to in-
sanity in the adult, and such weakness may be trans-
mitted in heredity.
362. Spontaneous Generation. — It is now the conunonly
accepted idea that living things develop only from similar
living things. However, this idea that all life comes from
life is rather recent. In former times, even scientific men
believed in spontaneous generation of life in dead, or not-living
matter. The sudden appearance of numerous earthworms,
REPRODUCTION OF ORGANISMS 413
frogs, mice, insects, etc., was explained by assuming that
they had suddenly originated spontaneously. When ac-
curate studies of life-histories began to be made, it soon be-
came evident that all the larger animals and plants originate
only from organisms like themselves. Until 1638 it was sup-
posed that maggots developed spontaneously from putrid
meat ; but in that year an investigator showed that maggots
never appear on meat which is screened so as to keep flies
from laying eggs on it. In some country regions it is still
believed that horsehair worms, which are often seen in stag-
nant pools, have developed from horsehairs, which they
superficially resemble. But that there is no connection be-
tween the worms and the hairs can be demonstrated by any
one who will place in bottles, stoppered with cotton, one or
a thousand horsehairs, and await development. Scientific
men long ago studied the structure and embryology of
horsehair worms and solved the mystery of their appearance
in pools, watering troughs, etc. The structure of the worm
is essentially the same as that of the other roundworms.
Its scientific name is Gordius, in allusion to its habit of twist-
ing into a tangle like the famous Gordian knot which Alexan-
der the Great cut with his sword. Its eggs develop into
minute larvae, which become parasites in insects, fishes,
frogs, and other animals. Later, the parasites escape from
these animals and develop into horsehair worms. Those
seen in horse-troughs have completed their life-history in
insects. Studies of habits have shown that the sudden ap-
pearance of earthworms is caused by flooding of their bur-
rows, and that they do not rain down ; and that crowds of
toads, frogs, mice, grasshoppers, etc., are due to very favor-
able conditions for developing eggs and to migrations into
new territory. All other such cases which puzzled even the
scientific people of a few hundred years ago have been ex-
414 INTRODUCTION TO BIOLOGY
plained so well that for more than a hundred years no scientific
man has believed in the existence of spontaneous generation
of any organisms higher than the bacteria. But until the
studies by Pasteur, supported by the work of the English
physicist Tyndall, between 1850 and 1870, it was believed,
even by men of science, that certain bacteria may develop
spontaneously in sterile bouillon and other foods. Pasteur
showed that if proper precautions are taken to make the foods
perfectly sterile, that is, to kill all Ufe in the test-tubes used,
and to prevent entrance of other germs, no organisms will
develop. In short, Pasteur showed that there is no evidence
that living matter originates spontaneously from not-living
matter. He did not show that it could not happen, for there
may be conditions of which we know nothing, as perhaps
existed at the first appearance of living matter on the earth ;
but he showed that the few cases in which some scientific
men of his time still believed had not been sufficiently tested
by accurate experiments. So far as concerns the origin of
new individuals of all known species of organisms now ex-
isting, we may siunmarize the studies which culminated with
those of Pasteur in the statement that all Uving things come
from Uving things, or all life from life.
APPENDIX
Table fob Refebencb to the "Applied Biology" and the
"Teachers* Manual op Biology"
Note. — In the first oolumn of figures, in italics, are the section
numbers of the "Introduction to Biology," and in the second
column, in Roman type, are the numbers of the sections of the
"Applied Biology" and the "Teachers* Manual" in which the same
topics are presented and from which, in many cases, the teacher
can get additional subject-matter and suggestions for pra-ctical
work. Where " 000 " appear in the second column there are no
special references to the " Applied Biology."
S3— 89
64—90
S5—91
57—93
S8—94:
59—95
60—96
61—97
6e—98
63-99
64—100
65—101
66—102
67-103
68—104
69—105
70—100
71—107
7S—10S
73—109
74—322
75—323
415
1^,5
26—27
e—1
27— 2S
5—2
28—29
4—3
29—30
5-6
30—65
6—7
31—66
7—S
32—67, 146
8—9
55—68
9—10
54—147. 148
10—11
55—69
11—12
5^—70
12-13
57—162. 163
IS—U
55—161
14—15
55—169
15—10
40—71
16—17
41—72
17— IS
42—73
18—19
43-181
19—20
44— no
20—21
45—000
21—22
46-82
22—23
47—83
23—24
48—84
24—25
49—85
25—26
50—86
416
APPENDIX
76—000
77—324
75—000
7^-000
50—325
51—326
82—000
55—332
54—333
55—334
5^—335
57—335, 133
55—35
55—36
50—37
91— SS
92—39
93—40
94—41
9S—42
96-43
97—44:
98—45
99—46
100—47
101-48
102—49
103—50
104—51
105—52
106-53
107—54
108—55
109—56-03
110—200
111—207
112— 20S
113—209
114—270, 271
116—213
116— 2S2
117— 2S3
118—284
119—292
120—308
121—309
122—370
123—371
124—372
126—373
126—374
127—375
128—370
129—377
130—378
131—379
132—3'&0, 381
755—382
i54— 383, 384
755—385
136-3180
137—387
138—388
139—389
140—390
141—391
142—392
143—393
144—394
146—395
146—390
147—397
148—398
149—399
160—400
161—401
162—402
163—403
164—404
166—405
166—400
167—407
168—408
169—409
160—410
161—411
162—412
163—413
164—414
166—415
166—410
167—417
168—418
169—419
170—420
171—421
172—422
173—423
174—424
176^425
176—420
177—427
178—428
179—429
180—430
181—431
182—^2
755—433
754—434
755—435
75^—436
757—437
755—438
755—439
750—440
757—441
75;?— 442
755—443
754—444
755—445
755—446
757—447
755—448
755—449
;?00— 450
;?07— 451
202—452
203—453
204—454
206—455
206—450
207—457
208—458
209-000
210—459, 460
;?77— 461
212—402
213—463
214—404
216—405
216—400
217—407
218—408
219—409
220—470
221—471
222—472
APPENDIX
417
;g;?5— 473
224— 4n^
226— ^Ib
226— ^IQ
227—477
228—478
229—479
230— 4S0
231— 4S1
232-000
233—249
234—246
235—2^
236—000
237— 2b4
238—255
239—250
240—257
241—259
242— 2G0
243—201
244—202
245-263
246—204
247—205
248— OOO
249—482 .
2S0—^S3
261— 4S4
262—^5
263—480
264—487
266—488
266—489
267—490
268—274
269—275
260—329
261—330
262—331
263—295
264—297
266-000
266—000
267-000
268-000
269—000
270—000
271—258
272—244
273—^5
274—240
276—240
276—247
277—2^
278—249
279—250
280—251
281—252
282—253
283—242
284—230
286—229
286—220
287—000
288-000
. 289—152
290—153
291—154
292—155
293—000
294—156
296—157
296—158
297—159
298—100
299—101, 162
500—163
301—100
302—107
303—168
304—170
306—171
306—179
307—180
308— (m
309-000
310—000
311—275, 276
312—277
313-281
314—291
316—295-297
316—302
317—315
318—320
319—327
320— 32»
5:^1—306-308
322—342
323—348
324—349
326—353
326—000
327—302
328—22, 28
329—267
330—237
331—218
55;?— 283-291
555—283, 308
554—218
336—74, 189 ff.
336—75, 191 ft.
337—76, 212 ff.
338—77
339—78, 135 ff.
340—79
341— 80, 141 ff.
54^^—81,145
545—000
544—223
54^—224
54^—225
547—57
545—58
54^—59
360— m
361—01
362—02
363—63
364—303
366—304
366—305
367—300
368—307
369—501
360—000
361—000
362—299
2b
INDEX
[The figures given refer to pages, not to sections.]
Absorption, 119, 175; by roots,
44,47.
Adaptation, flowers, 379; insects,
90.
Adenoids, 199.
iGsophagus, see esophagus.
Age, of animals, 17; plants, 21.
Air, breathed, 201. See oxygen.
Alcohol, 239-252. See also yeast,
and fermentation.
Algse, 39, 257, 316.
Amoeba, 137.
Amphibia, 360.
Angiosi>erms, 40.
Annuals, 27.
Antitoxins, 268.
Appendix, human, 164.
Arachnids, 291.
Arteries, 104; control of, 233.
Arthropods, 290-292.
Assimilation, 64. See also nutrition,
and metabolism.
B
Bacteria, 268-272 ; in hygiene, 273-
279; economic relations, 297-302.
Bathing, 231-233.
Bean plant, classification, 39 ; phys-
iology, 41-75; reproduction, 377-
384 ; structure, 24-39.
Bees, 88.
Biennial, 333.
Biology, definition of, 1 ; animal,
76-97; plant, 23-75; human,
147—222.
Birds. 363-365, 401.
Blood, uses of, 123, 129; human, 177.
See also circulation.
Blood-cells, 177.
Body-cavity, frog, 103; human,
151.
Body-wall, human, 151.
Bone, 114.
Botany, definition of, 1.
Brain, himian, 211.
Branching, of stems, 326.
Bread making, 316.
Breathing, animal, 15; chemical
tests for, 16, 20; human, 200;
hygiene, 224 ; plant, 20, 34. See
also respiration.
Bryophsrtes, 39. See moss.
Budding, 35, 324. See asexual re-
production.
Bulbs, 334.
Butterfly, 81.
Calorimeter, 190.
Cambiimi, 28.
Capillaries, 385.
Carbohydrates, 66, 156.
Carbon dioxide, in plants, 57; in
animals, 16 ; in human, 204. See
also excretion.
Caulicle, see hypocotyl.
Cell-division, 391.
Cell-waU, 26.
Cells, 26, 29, 111; use of foods,
187-198.
Centipede, 291.
Cephalopods, see squid.
Change, physical, 4; chemical, 5;
of matter, 4 ff .
Characteristics, of living things, 10.
Chemistry, 6.
Chlorophyll, 38; and plant food,
65-57 ; in Hydra, 325.
Chloroplast, 38.
Chromosomes, 392.
419
420
INDEX
Cilia, 132 .Fig, 50.
Circulation, animal, 123; plant, 75;
human, 181-187. See also blood,
and lymph.
Clam, 366.
Classification, principles and tables
39, 97 ; insects, 85, 96 ; man. 147.
Coagulation of blood, 179.
Cocoon, insect, 84.
Cod fish, 399.
Coilenterata, 145.
Coflfee, 254.
Colds, 226.
Colon, see intestine.
Colony, coral, 344 ; hydroid, 372.
Coloration, protective, of frog, 100 ;
of insects, 92-95.
Compounds, chemical, 7.
Conjugation, Paramecium, 369.
Conservation, of energy, 188-190.
Cooking, effect on foods, 176.
Codrdination, 129, 206. See also
nervous activity.
Coral-animals, 342.
Cord, spinal, 208.
Corn stalk, 32.
Corpuscles, blood, 178.
Cotyledons, 383.
Crayfish. 348.
Crustaceans, 347-349.
Cryptogams, 40.
Cultures, bacteria, 258-260; molds,
303-306; yeast, 313.
Cytoplasm, see protoplasm.
D
Death, animal, 17 ; plant, 21.
Decomposition, by bacteria, 301.
Development, embryonic, see em-
bryology.
Diaphragm, human, 200.
Diastase, 61.
Dicotyledon, stem, 328.
Diet, mixed, 196.
Diffusion, see osmosis.
Digestion, animal", 120-123, 138;
frog, 120; human, 159-175; Hydra,
143; hygiene of, 227; plant, 61.
Disease, bacterial, 265-269; and
insects, 282-286; and protozoans.
279-290.
Disinfection. 277.
Division. Amceba. 139; Parame-
cium, 369. See also cell-division.
Drugs, 254.
Dust, 225.
Ear, 215.
Earthworm, 346.
Eating, hygiene of, 227.
Echinoderms, 97, 354.
Economics, of plants, 297-340; of
animals, 340-367.
Ectoderm, 142.
Egg-cell, animal, 390; bean, 380;
fern, 388.
Elements, chemical, 6 ; in plants, 54.
Elodea, 19.
Embryo, bean, 384 ; fish, 397 ; frog.
395; fern, 389; mammal, 405;
plant, 380. See also reproduction.
Embryology, definition, 17. See em-
bryo, and reproduction.
Endoderm, 142.
Energy, in animals, 117; of human
body, 190 ; in foods. 188-191.
Entomology, 77.
Enzyme, 65, 139. See also under
digestion.
Epicotyl, 384.
Epidermis, plant, 28, 36. See also skin.
Epithelium, 110.
Erosion, and roots, 321.
E^phagus, human, 163.
Eugenics, 412.
Excess, in food, 228 ; in exercise, 238.
Excretion, human, 203 ; plant. 69.
Exercise, 237.
Eye, human, 212.
F
Fats, 157.
Fehling's reagent, 61, 155.
Fermentation, 312.
Ferns, 318, 387.
INDEX
421
Fertilization, animal, 391 ; plant,
380, 389.
Fertilizers, for soils, 54.
Fibro-vascular bundles, 30, 33.
Fishes, 358, 397.
Flies, 80, 285.
Flowers, 23, 40, 378-380; functions
of, 377 ; useful, 338.
Foetus, 407.
Foods, of animal, 15, 116; energy of,
190; human, 154-159; need of,
116; uses of, 187-197; of plant,
19, 54, 64. See also digestion, and
metabolism.
Food-storage, in roots, 320 ; in stems,
322 ; in leaves, 338.
Forestry, 337.
Frog, 14-17; anatomy, 99-109;
histology, 109-116; physiology,
116-130; embryology, 389-396.
Fruits, 40, 339, 381.
Functions, 24. See life-activities.
Fungi, 39, 302-316.
G
Ganglion, 209.
Gases, from organic matter, 11. See
also carbon dioxide.
Gasteropods, see snails.
Gelatin plates, 260.
Generation, spontaneous, 412.
Generations, alternation of, in fern,
389 ; in hydroids, 373.
Germ-cells, in heredity, 408. See
also egg-cell, sperm-cell.
Germicides, see disinfection.
Germination, 385.
Germs, see bacteria.
Gestation, 405.
Gill-slits, 400.
Girdling, 62.
Glands, salivary, 161 ; liver and pan-
creas, 165.
Grafting, 330.
Grasshopper, 77.
Growth, in living matter, 117 ; foods
for, 194 ; in stem, 326-330.
Gullet, see esophagus.
Gymnosperms, 40.
HsBmoglobin, see blood.
Hair, 219.
Heart, 182.
Heat, animal, 125; efifect on bac-
teria. 262; in foods, 194; in
germination, 386; from ski^, 220.
Heredity, 407.
Histology, of frog, 109-116 ; of plant,
26, 29, 36.
Horse-hair worm, 413.
Human biology, 147.
Hydra, 140-145.
Hydroids, 372.
Hydrophobia, 271.
Hygiene, 223-255, 278.
Hypocotyl, 383.
I
Ichneumon-fly, 88.
Immunity, 269.
Incubation, chick, 401.
Infection, 273-276.
Inheritance, see heredity.
Inoculation, protective, 271.
Insects, 76-97; in disease, 282-286;
useful, 319 ; injurious, 352.
Instincts, insects, 95.
Inter-cellular substance, 113.
Intestines, frog, 105; human, 163,
173.
Irritability, Hydra, 145; Parame-
cium, 135 ; plants, 71-73. See also
nervous activity.
Kidneys, frog, 107 ; human, 204.
L
Labor, division of, 136, 145.
Larva, butterfly, 81; frog, 396;
mosquito, 284.
Leaf, 35-39 ; useful, 337 ; variegated,
59.
Lenticel, 34.
422
INDEX
lichens, 310.
life-activities, animal, 14-18, 116-
130; in cells, 115; human, 147-
222; plant, 18-21, 41-75.
life-history, insects, 83-85.
light, artificial, on plant growth,
59 ; in photosynthesis, 57, 59.
liver-fluke, 345.
liver, human, 164.
Lobster, 347.
Locomotion, animal, 14 ; plant, 18.
Lungs, 199.
Lymph, 104, 180. See also circu-
lation.
M
Machinery, of life, 22.
Malaria, 279-285.
Manmials, 366, 403.
Man, biology and classification, 147 ;
structure and life-activities, 147-
222.
Mastication, 229.
Matter, orgiEinic, 3; three states of,
4.
Mesentery, 106.
Metabolism, 64, 129. See also growth,
repair, assimilation.
Metamorphosis, frog, 393; insect,
84.
Microbes, see bacteria.
Micro-organisms, see bacteria and
protozoa.
Milk, 168 ; bacteria in, 299.
Millipede, 291.
Mimicry, 93.
Minerals, in organic matter, 12; as
foods, 158 ; in soil, 54.
Molds, 257, 302-308.
Mollusks, 355-358.
Molting, insect, 83.
Monocotyledon, 40 ; stem, 32.
Mosquitoes, 282-285.
Mosses, 317.
Moth, 81.
Mouth-cavity, human, 159.
Movement, animal, 14; blood, 183;
plant, 18 ; respiratory, 200 ; stom-
ach, 167.
Mulching, 43.
Muscle, cells, Figs. 41, 42 ; hygiene,
237.
Mushroom, 55, 256, 308.
Myriopods, 291.
N
NaUs, 219.
Narcotics, 239-255.
Nerves, 208, 211.
Nervous activity, human, 206-212.
See also irritability. i
Nervous organs, 130, 208-212; hy-
giene, 234-236.
Nettle-cells, 143.
Nitrogen, as excretion, 204 ; .in foods,
195 ; in soils, 62.
Nucleus, 112.
Nutrients, 128.
Nutrition, animals, 128; human,
187-198.
O
Organisms, 3.
Osmosis, 45, 62.
Ovary, 105.
Overwork, 234.
Oviparous development, 395.
Ovipositor, 78.
Ovules, 380.
Ovum, see egg-cell.
Oxidation, 7, 124.
Oxygen, in blood, 202; in cells,
203 ; in plants, 66 ; in photosyn-
thesis, 67.
Oyster, 356.
Palate, human, 162.
Pancreas, human, 164.
Paramecium, 131-136.
Parasites, insect, 351 ; . malarial,
279; plant, 56, 320; protoioan,
279-282, 340; worms. 286-290,
345.
Pasteur, 414.
Pasteurization, 263.
Pepsin, 171.
INDEX
423
Perennial, 333.
Phanerogams, 40.
Pharynx, human, 162.
Photosynthesis, 57.
Phyla, 97..
Physics, definition of, 5.
Pl^siology, 22; animal, 78, 116-129,
134, 138, 143; human, 147-212;
plant, 41-75, 314.
Pith-rays, 329.
Placenta, animal, 406 ; plant, 382.
Plant, biology of, 23-75.
Plants, groups of, 39.
Pleurococcus, 370.
Plimiule, 384.
Pod, 381.
Pollen-grain, 380.
Pollination, 379.
Polyp, see corals.
Porifera, 146.
Pregnancy, 405.
Propagation, from roots, 322; from
stems, 335. See also reproduc-
tion.
Proteins, 60, 156, 157, 172, 195.
Prothallium, see fern.
Protoplasm, 29, 116.
Protozoa, 140, 281, 341.
Pruning, 64, 331.
Psychology, of digestion, 230.
Pteridophytes, 39.
Ptyalin, 171.
Pulse, 185.
Pupa, 81.
Pylorus, 105.
R
Regeneration, 373.
Rennin, 172.
Repair, 117, 194.
Reproduction, 17, 20 ; asexual plant,
374-377; asexual animal, 371-
374; frog, 389-396; seed-plants,
377-387 ; fern, 388 ; Paramecium,
133; amoeba, 139; Hydra, 371;
hydroids, 373; corals, 342; in-
sects, 83-85; fishes, 397; birds,
401 ; mammals, 403-407.
Reptiles, 362, 403.
Resemblances, in classification, 138.
Respiration, 127; human, 198;
plant, 66. See idso oxygen-supply,
and excretion of COi.
Retina, see eye.
Rings, annual, 328.
Root-cap, 26.
Root-hairs, 25, 45.
Root-pressure, 45.
Roots, 24-27, 320-322.
Rootstock, 333.
Root-tubercles, 25, 320.
S
Saliva, human, 161, 169.
Salmon, 398.
Sanitation, public hygiene, 278.
Sap, 58.
Saprophyte, 56.
Sciences, 3.
Scorpion, 291, 360.
Sea-anemone, 343.
Sea-weeds, see algse.
Secretions, digestive, 161-174; gas-
tric, 477; intestinal, 174; bile, 175.
Seedlings, 385.
Seed-plants, 23, 256, 318.
Seeds, 40, 319, 382-387.
Sense-organs, 212-217.
Sensitive plant, 18.
Sewage, bacteria in, 301.
Sieve-tubes, work of, 31, 62.
Skeleton, coral, 342; frog, 101;
human, 149 ; insect, 83.
Skin, human, 205, 217-222 ; hygiene,
231.
Sleep, human, 234.
Sleeping sickness, 281.
Slugs, 358.
Smallpox, 270.
Snail, 357.
Soils, and bacteria, 301 ; plant food-
materials in, 54; water in, 42;
erosion, 321.
Spermaphyta, see seed-plants.
Sperm-cell, 390 ; fern, 338.
Spider, 349.
424
INDEX
Sponge-animals, 145, 342.
Sporangia, see spore-case.
Spore-case, 303, 388. '
Spore-plants, 40.
Spores, 261. 303, 388.
Squid, 357.
Starch, in leaves, 58, 60 ; test for, 57 ;
stored, 65; formula, 67; as food,
156.
Starfish, 354.
Stem, 27-35; functions, 33; eco-
nomic, 322-337.
Sterilisation, 304.
Stimulants, 239.
Stimuli, e£Fect on plan^, 71.
Stipules, 193.
Stoma, 36. 51.
Stomach. 163.
Sugar, test for. 61, 155 ; as food. 156.
Sweat-glands. 219.
Ssmunetry, bilateral, and radial, 78.
Systems, of organs, 109.
Tadpole. 394.
Tape-worm, 286.
Tea, 254.
Teeth. 160.
Temperature, sense. 216.
Tests, for foods. 155.
Thallophytes, 39.
Tissues, animal, 110-116; plant,
human. 152.
Toad, 361.
Tobacco. 253.
Tongue, human. 161.
TonsU. 162.
Touch, 216.
Toxins, 268.
Trachea, human, 198.
Transpiration. 52.
Trichina, 289.
Tropisms. 73.
Tubers, 334.
Twigs, 323.
U
Unicellular animals, 131.
Ureter, 107.
Use of animals and plants (see eco-
nomics) ; in science study, 99.
Uterus, 404.
Vaccination, 270.
Valves, heart. 182 ; veins, 184.
Ventilation. 224.
Venus fly-trap, 19.
Vermes, see worms.
Vertebrates, 292-294.
Vinegar. 300.
Viviparous development. 395.
W
See
Waste, in living matter, 117.
also excretions.
Water, 10, 158; as excretion, 204
in germination, 386; in leaf, 50
need of, 41 : in organisms, 10
in roots, 44 ; in soil, 42 ; in stems,
48, 50 ; use of, in plant, 53.
Wood, 27 ; economic value, 335.
Work, see energy.
Worms, 286, 289, 346.
Yeast, 312.
YeUow fever, 282.
Yolk, 401.
Zoology, definition of, 1.
' I ^HE following pages contain advertisements of a
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Applied Biology
By MAURICE A. BIGELOW, Ph.D.
Professor of Biology in Teachers College, Columbia University, and
ANNA N. BIGELOW, M.S.
Teacher of Biology in Secondary Schools
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Economic Relations of Wood Structure; Bacteria; Usefiil Insects;
Injurious Insects; Economic Relations of MoUusks.
THE MACMILLAN COMPANY
Boston 64-66 Fifth Avenue, New Tork City Dallas
Chicago Atlanta San Francisco
Elementary Biology
By JAMES E. PEABODY
Hcftd of Department of Biology in the Morris High School
New York City, and
ARTHUR E. HUNT
Head of Department of Biology in Manual Training High School
Brooklyn
Elementuy Biology, Complete %iM
Elementuy Plant Biology 66
Elementary Biology — Animal and Hmnan • . . . 1.00
This book, a product of many years of successful teaching experi-
ence in secondary schools, emphasizes throughout the relation of the
science of biology to human wel^u-e. Both matter and method are
chosen with reference to this end. Structure and classification are not
n^lected, but they are made subordinate to function, because the lat-
ter is more significant for boys and girls of high school age. The
activities of plants and animals and their use by man are shown to be
closely related to human welfare, for example, the use of plants for
food and clothing; the relation of mosquitoes to malaria and yellow
fever and of flies to t3rphoid fever ; the work of bees and silkworms ;
the importance of birds as destroyers of insects ; the work of bacteria
as the friends or as the foes of man, etc.
The significant character of the book is further emphasized by sim-
plicity of style, abundant half-tone illustrations, chapter summaries, and
interesting laboratory exercises. Text and laboratory manual are com-
bined in one. In the appendix are found valuable suggestions for
teachers and pupils concerning laboratory equipment, special topics,
notebooks, and reference books.
THE MACMILLAN COMPANY
Bofton 04-66 Fifth Avenue, New Tork City Dallas
Chicago Atlanta San Frandaco
Elements of Zoology
By CHARLES BENEDICT DAVENPORT
Director of the Department of Experimental Evolution, Carnegie Institute of
Washington, and of the Biological Laboratory of the Brooklyn
Institute of Arts and Sciences, Cold Spring Har-
bor, Long Island, and
GERTRUDE CROTTY DAVENPORT
Formerly Instructor in Zoology at the University of Kansas
Cloth i2fno illustrated 508 pages list price $1,25
While this book is scientifically accurate it is not a dry, formal treat-
ise on zoology but an introduction to the study of animals, " It con-
tains an outline," say the authors in their preface, " of what we conceive
to be most important for the ordinary citizen to know concerning ani-
mals, and it will, we trust, stimulate a few to pursue the study of animals
as a life work with the aim of adding to human knowledge and discov-
ering important guiding principles.""
The content and method of treatment are such that the economic
value of the subject is apparent throughout. The content is every-
\vhere significant. It means something. Each chapter is not the
study of an individual animal alone, but of a type that illustrates a prin-
ciple and its bearing upon human welfare. Provision is made for lab-
oratory work to accompany the descriptive matter.
The significant character of the content, the simple style in which
it is written, and the numerous excellent illustrations, make it fascinat-
ing reading, the sort of book with which to go a-fishing for boys and
girls.
THE MACMILLAN COMPANY
Boston 64-66 Fifth Avenue, New Tork City Dallas
Chicago Atlanta San Francisco
Botany for Secondary Schools
By L. H. 'bailey
Of Cornell University
Cloth, i2mo, illustrated, 460 pages. List price, $1.2$
It is not essential nor desirable that everybody should become a botanist,
but it is inevitable that people shall be interested in the more human side
of plant and animal life. We are interested in the evident things of natural
history, and the greater our interest in such things, the wider is our horizon
and the deeper our hold on life.
The secondary school could not teach botanical science if it would ; lack of
time and the immaturity of the pupils forbid it. But it can encourage a
love of natiu*e and an interest in plant study ; indeed, it can originate these,
and it does. Professor Bailey's Botany has been known to do it.
In the revision of this book that, has just been made, the effective simplicity
of the nature teacher and the genuine sympathy of the nature lover are as
successfully blended as they were in the former book. Bailey's Botany for
Secondary Schools recognizes four or five general life principles : that no
two natural things are alike ; that each individual has to make and main-
tain its place through struggle with its fellows ; that " as the twig is bent
the tree inclines"; that "like produces like," and so on. From these
simple laws and others like them Professor Bailey proceeds to unfold a
wonderful story of plant individuals that have improved upon their race
characteristics, of plant communities that have adopted manners from
their neighbors, of features and characteristics that have been lost by
plants because of changed conditions of Ufe or surroundings. The story '
vibrates with interest.
The book is, moreover, perfectly organized along the logical lines of
approach to a scientific subject. Four general divisions of material insure
its pedagogical success :
Part I . — The Plant Itself ;
Part II. — The Plant in Its Relation to Environment and to Man;
Part III. — Histology, or the Minute Structure of Plants ;
Part IV. — The Kinds of Plants, including a Flora of 130 pages.
THE MACMILLAN COMPANY
Publishers 64-66 Fifth Avenue New Tork
BOSTON CHICAGO ATLANTA DALLAS SAN FRANCISCO
To avoid fine, this book should be returned on
or before the date last stamped beJow
75f
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